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The Reason We Haven’t Directly Detected Dark Matter

• darawk 8y ago
  Can someone that knows a bit more about this explain why we believe that this dark matter substance must exist and not simply that we have an incorrect model of gravitation?
  • krapp 8y ago
    The article describes the rationale, as does the "observational evidence" section of the Wikipedia article on dark matter[0]. tl;dr no alternative models for gravity can sufficiently explain all of the data, but general relativity and "dark matter being particles" does, so that's what we're going with. [0]https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...
  • deleted 8y ago
    [deleted]
  • kamaal 8y ago
    For a very simple reason that, you have to start explaining things from the concepts you already know are true.
  • cygx 8y ago
    Because such a minimal change to the model ("there's just some more stuff we can't see") fixes a number of problems in one fell swoop (off the top of my head, galactic rotation curves, observations of gravitational lensing, a cosmological model compatible with obervations of CMB fluctuations, structure formation, cosmic evolution). Theories of modified gravity can fix those individually, but as far as I'm aware, no alternative has been shown to be viable once taken in combination.
  • dwaltrip 8y ago
    No one has been able to come up with a compelling alternative model of gravity. All the attempts so far have largely failed (their have been many), as the proposed theories break in one or more critical ways. I think their may be a small number of languishing models that aren't totally disproven but don't look very promising. But I'm not sure.
  • InclinedPlane 8y ago
    In short: "because science", quite literally. Astronomers are scientists, they don't make shit up just out of boredom, they only believe theories when they've withstood rigorous testing through observational evidence. It's important to understand the history of dark matter / "missing matter". It's a multi-decade history that originally started out with a small amount of intriguing but seemingly persistent observational data that couldn't be explained easily (namely that when you measure how much galaxies weigh by looking at how fast the stars are orbiting that figure differs a lot from the weight you get by calculating up the contribution from all the stars and gas and dust and whatnot that we can see (the "light" matter)). In response a huge number of different "theories" (more properly hypothesis) were brought up to try to explain the mystery, while new ways of studying the problem were thought up as well. And over the many years after the initial evidence came to light (in the '70s) considerably more evidence came to light from a wide diversity of observations. I won't list them here but I'll point out that the wikipedia page on dark matter has a good run down. Anyway, the fascinating twist to the story here is that as this evidence came to light it started eliminating various hypotheses about this missing matter until finally only one was left: the current theory of dark matter. It's not just a crazy idea, it's a crazy idea that fits all of the evidence when nothing else did. So don't look at the WIMP dark matter theory as though it's just some half-baked idea, it's a hard fought veteran of numerous campaigns to kill it, but it just keeps going because by all the evidence it seems to be the only theory that explains reality. As to specifically the "modified gravity" theories that compete with dark matter, they have a hard time explaining several observed phenomena, especially things like the famed "Bullet Cluster". There are a few examples where we can observe collisions between galaxy clusters and through different techniques we can map out the distribution of stars and of gas and of mass in the collision. What we observe in the Bullet Cluster as well as some others is that the gas is in a completely different place than the stars (because stars in galaxies mostly pass through one another whereas gas clouds squish together) and the center of mass of the stars of the galaxies is separated from the center of the non-visible mass of the galaxies. This absolutely cannot be explained by any of the modified gravity theories.
• blueprint 8y ago
  > we know how gravity works haha
• xVedun 8y ago
  I'm guessing that "guessing in the dark" is the way experiments for particles in the standard model have been done for a while now. I would assume that a single success would set off a chain reaction as the author said.
  • analog31 8y ago
    I'd say it's not completely in the dark. The experiments try to test theories that suggest where to look. For instance if a theory predicts a previously un-discovered particle, with some rules for how it interacts, then you can refine your search based on those rules -- what range of masses you're looking for, what decay patterns, and so forth.
• allcentury 8y ago
  Really well written article, especially as someone who is interested in the topic but not an expert.
  • bensonn 8y ago
    As I was walking the dog last night I was thinking about SEO and how even the "experts" don't really know Google's secret sauce or how the recipe will change. There are best-practices that seem to work but with Google's algo hidden and changing it seems funny to be an expert. But certainly they know more than their clients. That led my thoughts to dark matter. Expert: I am an expert on Dark Matter. Me: What is it? Expert: I have no idea but I think it exists. Me: Ok, you must be really smart. My glib little dialog contains no sarcasm. No doubt they are very smart. Expertise is measured differently in different fields. Expert on Oak Island knows all the rumors and theories but not where the treasure is. Expert flat-earther knows all the wrong facts. Expert politician might know .0001% due to the vastness of government. Experts on religion know the experts in other religions are wrong. Expert MLB hitters fail more than succeed. I guess it gives me hope I may one day be an expert at something.
    • ddmd1 8y ago
      I think the definition of "expert" can be pretty ambiguous. We have one type of "expert", the MLB hitter who is an expert because he is more skilled than others at hitting. And we have another type of "expert" who is an expert because she knows more about something than others. These are two very different types of "experts" but what makes them experts is their skill/knowledge relative to the general populance rather than skill/knowledge relative to some absolute.
      • raattgift 8y ago
        Even the best batter in the league goes to batting practice, studies what other good batters do, takes advice from coaches, some will likely study sports science literature themselves (rather than relying on coaches doing that for them). Batters will always want to think of ways to improve their mechanics, and will always want an "edge" over pitchers, and pitchers vary in their techniques. The great hitters from decades ago did not have access to video footage of themselves, their rivals, and opposing pitchers, for example. So while some of them were exceptionally skilled at hitting baseballs (and not just home runs) (and also running bases and being competent in field positions usually) they were not really experts for want of a body of rigorous literature. Conversely, one can of course have sub-major-league skills but enormous expertise -- there are batting coaches and sports science academics after all, and even popular analysts. And sports skills decay with age. Nobel-prize-winning scientists can get senile dementia too; sadly that not just wrecks their skills, it also wrecks their expertise as they forget much of what they've read and studied.
• Entangled 8y ago
  We have a room filled with one foot of iron, one foot of sand, one foot of water, one foot of air, and some empty space filled with god knows what needs to be filled in order to balance the gravity equations. There is one group of people looking for particles less dense than air to fill the rest of the room. I am in the group that's looking at the other end for particles denser than iron, and I firmly believe they are contained in black stars (wrongly named black holes).
  • bsaul 8y ago
    aren't black holes mass already computed accurately ?
  • stallmanite 8y ago
    Is there anything on Arxiv I could read to learn more about your approach?
  • InclinedPlane 8y ago
    You can "firmly believe" in whatever you want. That's not science, that's belief. The evidence points towards the existence of dark matter very strongly, and the evidence has eliminated the possibility of dark matter being made up primarily of black holes. Which, incidentally, do exist and are not "black stars" (the gravitational wave detections of merging black holes are the strongest evidence yet of the existence of black holes and don't leave wiggle room for alternate theories). If you don't form your beliefs based on where the evidence leads you then you are not following science, period.
• tychomaz 8y ago
  Because it doesn’t exist. Dark matter is a fudge factor to prop-up a theory that astrophysics is too afraid to put to rest.
  • user812 8y ago
    Indeed. Physics is wrong on many fundamental levels, but everytime physicists find fundamental errors, they propose another entirely theoretical layer of complexity, with the "benfit" that no one is able to practically refute it.
    • williamdclt 8y ago
      It's not adding a layer of complexity, it's peeling off a layer of abstraction. With the "benefit" that it describes better what we observe and that it allows to successfully make new predictions. What's your alternative to the scientific method exactly?
      • user812 8y ago
        100 or so years ago physics went into the wrong direction, ignoring people like Tesla who argued for the aether model.
    • geertj 8y ago
      > Physics is wrong on many fundamental levels Physics, when practiced according to the principles of the scientific method, is never wrong (or right). Physical laws try to explain the world around us using mathematical models. Those models can be tested over a certain domain of experimental parameters D. A physicist, when speaking accurately, would never say that a physical law L is correct. They would say that L correctly describes our experiments on domain D. New or more accurate measurements can expand or shrink D, and in the worst case D can become empty. This constant refining is the essence of physics and the scientific method. Arguing that physics is wrong implies a lack of understanding of what physics actually is.
      • 8y ago
• perlgeek 8y ago
  My attempt to summarize the reasons would be: We haven't directly detected dark matter because we don't really know what we are looking for, and there are only a few things we can search for. Dark matter might not be structured in a way we can investigate with current technology. Also, dark matter doesn't interact much with regular matter, which makes the search even harder.
  • beefield 8y ago
    > Also, dark matter doesn't interact much with regular matter, which makes the search even harder. This is (one of the many places) where I get lost. It has mass, so it by definition interacts with anything with mass? Are these particles supposed to be so small and so rare that they can't be measured even at the scale of solar system? (How much dark matter would be in the solar system? What would be the average density of the dark matter? what would be the mass of single dark matter particle?) Or what do I miss here?
    • analog31 8y ago
      As I understand it, gravity is the interaction between things with mass, but it's a weak interaction, thus hard to detect on a particle by particle basis. You might be able to detect if there's an extra potato in a bag because a potato is a whole bunch of particles. But identifying that just a few of those particles are not the regular stuff -- protons, electrons, etc. -- by weighing the bag, would be difficult.
      • beefield 8y ago
        But you do not need to measure it particle by particle. Just because those have mass, there should be a bunch of dark matter particles hanging around with earth. And given that we have quite good idea what earth consists of, there should be a discrepancy in some of the measurements that use earth's mass against the mass we have from our understanding of earth's composition. Unless, of course, the extra mass of earth due to dark matter is calculated in e.g. kilograms. That's why I would like to know the expected density of the dark matter.
• _ph_ 8y ago
  Nice article, especially showing how much experimental data about dark exists. Dark matter isn't just a crazy concept to "fix" gravity. As nicely described in the article, there are several experiments clearly showing some invisible matter. We know a lot about it, except which unknown particle causes it.
• throwaway9258jj 8y ago
  Author is a soyboi and lacks female attention.
• skdjii 8y ago
  Another reason may be that dark matter doesn't exist. If the universe consists of 85% of the stuff it would seem that it should be easy to find. Occams razor should apply as usual.
  • danharaj 8y ago
    That's not Occam's razor. The razor applies to theories with identical predictions and tells you to pick the simplest one. I'm not sure how you got your inference from Occam's razor.
  • antidesitter 8y ago
    > If the universe consists of 85% of the stuff it would seem that it should be easy to find. How did you infer this? We are bathed in gravitational waves but they weren’t “easy” to find. > Occams razor should apply as usual. So what would be your razored explanation for the gravitational anomalies that point to the presence of matter we cannot see?
    • roenxi 8y ago
      Our understanding of gravity might be completely wrong. If 85% of the gravity is unexplainable that seems like a pretty safe bet. Maybe it just behaves very differently at galactic scale. Much like the aether, it seemed most likely that light traveled through a medium until it turned out that it probably doesn't.
      • furgooswft13 8y ago
        The aether stuff confuses me. We replaced light traveling through some medium, the aether, with light traveling through a photon field which permeates all of space, among many other fields. That sounds like an aether by another name. Yet now we say aether theories of the turn of the 20th century types are debunked and everyone then was wrong. Yes the medium of a field is very different in nature than any classical ones (it's quantum after all), but the concept overall seems similar. Even when aether theory was in vogue it was known that the light medium would have to be very different than that of a gas or liquid (waves traveling without resistance). So maybe this is being pedantic, but I just don't see how a field is not a medium. It seems like we found the medium and described it, then gave it a new name and turned around and laughed at our physics forebearers like they were ignorant geocentrists (slight exaggeration).
• lottin 8y ago
  So, basically physicists have deduced two facts, and these facts don't agree with one another. From this they conclude that there must be something else in the universe, something unseen and undetectable, that they call dark matter. But what about the other possibility, which seems far more reasonable to me, that either one of the facts is wrong?
  • InclinedPlane 8y ago
    It is not "one fact", it's a whole string of evidence (as partly explained by the article). The evidence strongly supports the dark matter theory and the evidence has consistently eliminated every single other theory attempting to explain the phenomena.
  • dwaltrip 8y ago
    Is it possible that you haven't had the training to understand the weight of the evidence and theory that led to this conclusion? Perhaps some humility is necessary here?
• std_throwawayay 8y ago
  If we can't detect it or disprove it, it doesn't even belong to science. It belongs to the same realm as witches and demons - until some smart physicist devises a way to actually detect it.
  • InclinedPlane 8y ago
    We can disprove it, any one of the many, many lines of evidence that have been collected so far could have disproved the WIMP/CDM theory of dark matter. They haven't. Instead they've eliminated all of the other alternatives. That's science.
  • enkid 8y ago
    You have to have a theory to test before you can test it. If they are stating as fact something that hasn't been proven, that's a problem. If they are testing the assumptions which closest matches the data, that is science.
  • beojan 8y ago
    We can detect it, gravitationally. The Bullet cluster is pretty incontrovertible evidence that dark matter exists. It's detecting it through other interactions that's proving extremely difficult.
  • djsumdog 8y ago
    The Higgs field was in the realm of theory for a long time. The maths matched up, but it wasn't until the LHC that scientists had the instrumentation to even look for it. Two other collides at the time through they were getting close, but it turns out the Higgs Boson wasn't anywhere near where they thought it'd be. It took a long time and a lot of work, but the experiments are reproducible (and they were reproduced several times to ensure they had found the Higgs and to learn more properties about it).
• tomkat0789 8y ago
  This may be a dumb question, but could dark matter just be regular matter, like planetoids? What if interstellar space had a light sprinkling of things like Oumuamua? On an interstellar scale, maybe this could create the lensing effects and explain some other phenomena. Is it unreasonable that 5/6 of the mass of the universe is like the stuff that makes up planets, but isn't lit up as brightly as a star?
  • dTal 8y ago
    Not a dumb question, but obviously they've thought of that: https://en.wikipedia.org/wiki/Dark_matter#Baryonic_matter https://en.wikipedia.org/wiki/Dark_matter#Baryonic_matter
  • InclinedPlane 8y ago
    This is the "MACHO" (massive compact halo objects) theory, which has been tested and disproven. The missing mass can't be gas or dust clouds because that would have certainly observational traces which we have looked for and don't exist. It can't be planetoids because that would cause a frequency of gravitational micro-lensing of stars in other galaxies that we do not observe.
• deleted 8y ago
  [deleted]
• tomp 8y ago
  The simplest “proof” of dark matter - Kepler’s Laws dictate that objects with smaller orbits (closer to center) move faster (in terms o fangular velocity). Our solar system works like that. Our galaxy doesn’t - its spiral shape indicates that the angular velocity varies minimally with radius.
  • ajross 8y ago
    The spiral structure isn't fixed to specific matter, it's a wave moving "through" the galaxy. But broadly yes: you can look at orbital velocities of galactic matter directly (via doppler shifts of their spectra) and make a map of the mass distribution in the galaxy. And then you can add up all the mass we can see as stars, and they don't remotely add up.
  • chongli 8y ago
    This carries the implicit assumption that all forces fall off monotonically with distance. Maybe gravity doesn't? Maybe it has a curve similar to a meteor impact crater: raised in the middle, a deep valley, and raised at the outer edge? Of course, with more evidence, such as the observations of the Bullet cluster, these simple explanations fall apart. But at any rate, it's not enough to take one observation and assume it holds at all scales. Look at tiny water droplets. They don't behave at all like large bodies of water.
    • CydeWeys 8y ago
      Gravity behaving like that would be a lot weirder than there simply being some types of mass we aren't able to detect yet. Physics has had a long history of predicting particles that hadn't been detected yet, and then actually going out and finding them. See neutrinos, the Higgs boson, etc. General relativity being wrong would be waaaay weirder.
      • kamaal 8y ago
        >>General relativity being wrong would be waaaay weirder. If I'm not wrong, Newton's laws can still be held to be true in some cases even inside GR? Some where in the past, some one could come up with a new model which could show us new truths, and yet could contain GR to be true in some cases. In GEB, Hofstadter show us part of the ways so see intelligence emerging from noise and garbage, is to understand when you design a system, you start with axioms, truths and rules that are true in a system, could be total garbage in another. Models are what we use to understand things, That's not how things are.
• krapp 8y ago
  I like how we have an article explaining the rationale behind dark matter and the scientific principles that lead to the conclusion that it exists, but most of the threads here are simple declarations that no, they must all simply be wrong or not considering some simple alternative. Can people maybe, just maybe not assume that scientists are idiots or, as once dead commenter puts it, charlatans? If you disagree with the consensus, then offer some alternative more convincing (and more useful for civil and interesting discussion) or dispute the article rather than than just writing it all off as "fake science."
  • dTal 8y ago
    I was about to post something similar. Before you act on the urge to go pontificating about how the scientists have clearly got it wrong for this reason or that reason, ask yourself - do you really think they haven't thought of that? Don't you think that, if there is mileage in your idea, there would certainly be scientists pushing it? Addendum: Science (and more generally, specialization) requires that we trust that that the experts in the areas outside of our own chosen fields do in fact know what they are doing. It's become less fashionable to do so lately (one politician in the UK famously said a few years back that "the people of this country have had enough of experts"), but that is to our cost - the extreme end of this is flat-earth conspiracy.
    • maxxxxx 8y ago
      I think the backlash against "experts" comes from the fact that way too many things get mixed into "science". When you look at fields like social sciences, nutrition and economics there are a lot of "scientists" that get publicity for results of very undisciplined work. They don't crosscheck their result for contradictions and often just seem to look for confirmation of their own biases. This contrasts to most physics I read about. Physicists seem to make a big effort to reconcile all their measurements and if they contradict they admit that and say "we don't know yet". So physics and other hard sciences suffer from being put together with a lot of other bad science in the public.
      • TheOtherHobbes 8y ago
• OneWordSoln 8y ago
  The missing 5/6ths of the universe is within the other five vibrational dimensions (one of which houses anti-matter) that do not directly interact with the energy and matter of our layer of the onion in normal conditions. At high enough energies, we get brief crossover and can thus detect particles such as antielectrons, but they quickly revert to their own dimension. IIRC, I believe Paul Dirac mentioned this as a possibility but was quickly ignored. (It may have been another physicist of his time.) While not directly interacting with our layer, it does somehow contribute to the totality of gravitational attraction. How? I haven't a clue. And, while we're here, dark energy is simply the added intrinsic energy that keeps the universe from collapsing back in upon itself, for the simple reason that creation is intended to exist as more than a point particle. I know this will get downvoted to oblivion so I will add that our soul inhabits one of those other layers of the onion yet is trapped within our physical body while we are conscious. Because of our remedial-level of spiritual development (we are still stuck in mammalian competition with human cooperation being our potential), we are not capable of consciously exploring the universe with our soul, and because we do not understand our own human structure, we are not open to comprehending the structure of the universe in all its majesty. The most important point we fail to understand is that we are designed to interact with the information matrix we exist within and can query it directly once we reach a certain level of spiritual development, but one must first admit to our human nature to begin that journey of becoming intimate with our Unfathomable Creator and this magnificent, mysterious creation. The Way goes in. -- Rumi
• zvrba 8y ago
  So my takeaway from this article is: where there is a concentration of mass, there is dark matter. (Quote: "According to models and simulations, all galaxies should be embedded in dark matter halos, whose densities peak at the galactic centers.") Wiiild speculation: perhaps mass is actually a complex number with real and imaginary parts (analogous to how quantum mechanics describes fields), and what we're able to measure directly (by weighing) is the real part. The imaginary part ("dark matter") is some yet unknown interaction with gravitational force and what we measure indirectly with gravitational lensing is the complex magnitude.
  • panic 8y ago
    > So my takeaway from this article is: where there is a concentration of mass, there is dark matter. Not always: https://www.nature.com/articles/nature25767 https://www.nature.com/articles/nature25767
• EamonnMR 8y ago
  We detect most matter through its interaction with electromagnetism-we touch things and see things, essentially. I think the "aha" moment is when you realise that something invisible to light would also be incorporeal.
• stevespang 8y ago
  >The motivation. We know how stars work, and we know how gravity works . . . . You say we know how gravity "works", but we still cannot unify general relativity and quantum mechanics to accomodate gravity. Why don't you start by admitting that all the observations of the stars and galaxies beyond out solar system are events which are long past by the time the electromagnetic radiation (light) reaches us, often millions of years or even longer.
• coleifer 8y ago
  There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy. Just because we can't see it or measure it doesn't mean there's nothing there. Fascinating to get some insight into how scientists concluded such a thing exists.
  • Retra 8y ago
    We can measure it.
• jumpywizard 8y ago
  Anyone trying to use machine learning to make a "black box" mathematical framework? Feeding known measurements as "learning" (including known anomalies and discrepancies)? Using techniques such as deep learning for program synthesis. https://www.microsoft.com/en-us/research/blog/deep-learning-program-synthesis/ https://www.microsoft.com/en-us/research/blog/deep-learning-...
  • pp19dd 8y ago
    Yep, it's been done before by Cornell researchers, in 2009. You provide a bunch of data to a program and it hums for awhile and derives some equations from the observation. In the story written around the time, it took positions of a pendulum swing and derived laws of motion from it. See https://www.wired.com/2009/04/newtonai/ https://www.wired.com/2009/04/newtonai/ (and lots of other articles around the time.) You could feed a spreadsheet into and then it would iterate for awhile, converging toward most accurate equations describing the system. Or in my case, diverging from what I was sure was the solution. It was a hit and miss with most data (honestly, an eyeball and a few braincells did better with some data sets), but all the same I used it to lazily approximate position equations for some programs I was writing. The desktop program was called Eureqa which then got ported to the cloud for obvious capacity increases, and relabeled into a company/product called Nutonian. Most recently bought by "DataRobot" and it's now being sold for sales optimizations. Because science.
• mcguire 8y ago
  "The theory. This tells us that around every galaxy and cluster of galaxies, there should be an extremely large, diffuse halo of dark matter. This dark matter should have practically no “collisions” with normal matter — upper limits indicate that it would take light-years of solid lead for a dark matter particle to have a 50/50 shot of interacting just once — there should be plenty of dark matter particles passing undetected through Earth, me and you every second, and dark matter should also not collide or interact with itself, the way normal matter does." I've always heard that the lack of interactions was an observation, not a deduction: we can't see it, therefore it doesn't interact.
  • sytelus 8y ago
    When matter particles interact they release energy that we can detect in form of x-ray or light. When dark matter particles interacts with matter or dark matter, they don’t seem to release any energy. Their only giving away clue is gravitational effect. If dark matter does exist and can be directly detected then we are looking at very new physics.
• slacka 8y ago
  Despite recent some initial evidence to the contrary,[1] I'm still a fan of the idea that dark mater is actually concentrations of matter/energy outside of our universe. If string theory is true, gravity does leak into the multiverse, and we survive the Great Filter, then someday we may be able to use gravity to communicate universes outside of our own. https://www.sciencenews.org/article/gravity-doesnt-leak-large-hidden-dimensions https://www.sciencenews.org/article/gravity-doesnt-leak-larg...
  • The_rationalist 8y ago
    Then why would dark matter be detected in galaxies but not in void space ? Why would those external world matter/energy be spatially syncrhonised at the place of galaxies ? Ockham Razor to the rescue.
    • matt_kantor 8y ago
      I'm just a layman, but the parent said "if string theory is true, gravity does leak into the multiverse". Given that I'd imagine the clumpiness of our universe could influence the clumpiness of nearby universes (and vice versa).
      • slacka 8y ago
        Exactly. Not all galaxies have the same % of dark matter.In fact some have none.[1] Maybe those with little to none, formed by random fluctuations, while others formed because of the attraction to dark matter in their vicinity. https://www.smithsonianmag.com/smart-news/galaxy-without-dark-matter-shakes-astronomy-180968628/ https://www.smithsonianmag.com/smart-news/galaxy-without-dar...
• zoomablemind 8y ago
  Excuse a layman's observation here. The dark matter problem rings as yet another mass-related anomaly we stumbled upon previously but on a nuclear scale. "Mass defect" [1], as appropriately named, is observed on a nuclear scale (whole nucleus mass is less than mass of nucleons that the nucleus is composed of). It is explained with binding energy required to keep the individual nucleons of a nucleus together. That is an 'invisible' mass that gets subtracted from the 'free' mass of nuclear particles as measured. Any possibility to project such analogy onto a galactic scale? [1]: https://en.m.wikipedia.org/wiki/Mass_defect https://en.m.wikipedia.org/wiki/Mass_defect
  • gus_massa 8y ago
    In the Mass defect case, it's not an invisible mass what it is substracted. When some nuclei react to produce another nuclei, the energy of the mass defect is emitted as a gamma ray or as the energy of a fast electron/positron or something else. But all the mass can be found somewhere. It is only missing because you can't keep the gamma ray inside the balance. There should be a similar effect in a galaxy, because when the stars/gas/whatever get closer they have a small gravitational binding energy. I didn't do the calculations, but I'm sure it is so tiny (relatively) that you can safely ignore it. For comparison, in the Helium nuclei the difference is only 0.8%, in a galaxy it is (I think) abysmally smaller because the gravity is smaller than the strong force and the stars are much far away than the nucleons. But the dark matter is much bigger than the normal mater, something like x5 or x6. So it's not possible to explain the dark matter with a something that has a tiny effect.
• burlesona 8y ago
  This strikes me a lot like “flatland”, ie. there’s something there but we’re not equipped to perceive it. Our experiments are mostly like extreme extensions of our own senses - seeing and touching things. But could something exist entirely within a dimensional space that we can’t touch or see? And if so how would we ever figure out how to detect it ... and even more meta, what if it was impossible for us to perceive the detection? It’s all really fascinating.
  • dwaltrip 8y ago
    Well, it has a gravitational effect, so it definitely exists inside of our normal spacetime. Scientists have explored alternative models of gravity or spacetime with additional spatial dimensions and things like that. As far as I know, none of them are very promising. There was an interesting PBS space time video about one example recently: https://youtu.be/3HYw6vPR9qU https://youtu.be/3HYw6vPR9qU Of course, it is definitely interesting to think about :)
    • burlesona 8y ago
      Interesting, I didn’t know that! I guess for me the analogy is to the sphere in flatland being perceived as a circle - we can see evidence of dark matter but so far we can’t really detect or measure it directly. I wonder if the gravitational effect is the only clue that we’ll find inside “normal” spacetime. The most exciting thing about that is the idea that the hunt for dark matter might eventually lead to an understanding of the “rest of the universe” where we would otherwise never have a hint of its existence.
  • stephengillie 8y ago
    The problem is that gravity, like EM, follows an inverse-square rule. If gravity had a 4th dimensional component, we would expect to see an inverse-cube situation. The equation is: 1/(1-dimensions) Youtube video that explains this in depth: https://www.youtube.com/watch?v=3HYw6vPR9qU&t=726s https://www.youtube.com/watch?v=3HYw6vPR9qU&t=726s
    • 8y ago
• The_rationalist 8y ago
  « On 25 August 2016, astronomers reported that Dragonfly 44, an ultra diffuse galaxy (UDG) with the mass of the Milky Way galaxy, but with nearly no discernable stars or galactic structure, may be made almost entirely of dark matter.[6][7][8] » Meta-source: https://en.m.wikipedia.org/wiki/Dark_galaxy https://en.m.wikipedia.org/wiki/Dark_galaxy
  • The_rationalist 8y ago
    Isn't this a proof of the invisibility of dark matter ?
    • lev99 8y ago
      It's an observation that fits our theoretical models when dark matter is assumed. It's indirect proof. The indirect proof is so overwhelming strong that any rational person will concluded with reasonable certainty that dark matter is real. Directly detecting a dark matter particle will give us more data than "dark matter is real".
• mixmastamyk 8y ago
  Believe the issue people have with “dark matter” is really about the term, it points to a so-far unprovable solution as a goal rather than a problem. It elevates a hypothesis to already solved, just need evidence. If they called it the “galactic paradox” it would be more easily recognized as a problem to work on. You can downvote but it only proves the perception problem, not wanting to hear it doesn’t help.
• angel_j 8y ago
  How do you square the following notions? a) dark matter does not interact with measurable matter b) proof of dark matter's existence derived by the measurable, relative, interaction of mass and energy
  • dbt00 8y ago
    Summarizing on the fly, so necessarily wrong in details: Regular matter interacts via the four forces: strong interaction, weak interaction, electromagnetism and gravity. Strong interaction: binds fundamental particles together to make atoms. In every day terms, it’s what makes mass, mass. Weak interaction and electromagnetism: causes radioactive decay and EM radiation (photons). In every day terms, it’s how we get light, radio, cell phones, etc. Also nuclear power. Gravity: attraction of mass across great distances. It’s how we stick to the earth and why the earth circles the sun. There are particles that don’t act through forces but are affected by them: Photons don’t have mass so don’t cause gravity but are affected by it. Neutrinos don’t have an electromagnetism effect so they can’t be seen at all only measured in very rare weak interactions. Dark matter: No strong interaction so not a part of atoms. No EM interaction so we can’t see it blocking light. No weak interaction that we have observed. However they do cause gravity, so we can see that. All of the proofs of existence are via gravity and relate only to mass — we can’t observe any other properties.
    • vbezhenar 8y ago
      How is it even possible to detect dark matter, if it doesn't have EM interaction, weak interaction and strong interaction? Probably some ultra-sensitive gravity detector which could detect a single particle? Sounds like impossible for current technology. So we could have those dark matter particles orbiting with crazy speeds around our planet, sun and everything else without any chance to reliably detect them. But still there are experiments to detect them, does it mean that scientists expect some kind of interaction, may be rare?
      • gmjoe 8y ago
        That's exactly it. Nobody knows if it's impossible or not, but there's no way to find out unless experiments at least try, to whatever effort/cost we deem reasonable.
• angel_j 8y ago
  I'm not an astrophysicist, but how do we know we're not measuring overlapping folds of relative spacetime, and erroneously counting the same observable matter more than once?
• mkagenius 8y ago
  Can dark matter be reminiscent of past, a time travelling effects of matter..
• Michael_Groom 8y ago
  You might want to check out a very interesting non-linear spinor theory[1] which contains a chapter on dark matter and dark energy analytically explaining the formation and inner structure, as well as the ratio of dark matter and dark energy throughout the development phase of the universe. Page 29 lists all components of DM/DE and normal matter. [1] http://norbert-winter.com/wp-content/uploads/2018/07/2018_02-16_UC-G-en_norbert-winter_der-universums-code_das-erzeugungssystem-des-vollstaendigen-universumsgesamtprozesses.pdf http://norbert-winter.com/wp-content/uploads/2018/07/2018_02... Full theory can be found here: http://norbert-winter.com/wp-content/uploads/2018/02/2017-03-17_UC-AOS-en_norbert-winter_the-universe-code_the-unified-composition-and-order-system-of-the-universe.pdf http://norbert-winter.com/wp-content/uploads/2018/02/2017-03...
• richardjennings 8y ago
  As an uneducated layman I have 2 questions I have failed to understand that continue to confuse me. 1. How can relatavistic time resolve to a instant for which the mass in the universe applies? 2. Why does spacetime not have wave like tensional disortions?
  • hcs 8y ago
    Not following your first question, but isn't 2 the recently-observed gravitational waves?
• thx4allthestuff 8y ago
  Super weird food for thought, but I used to think about the universe a lot as a kid, and back then we were under the assumption that the expansion of the universe was slowing. At some point that viewpoint changed and we now believe it is accelerating, hence the emergence of so called dark matter. Anyway, this led me to envision a fourth dimension, a sphere. Imagine that our universe began at any arbitrary point on the inside of this sphere, and then orient the sphere so that we are at the bottom (like a penny inside of an inflated balloon). Now imagine (don’t believe, just imagine) that our universe is expanding at a constant rate. As we approach the equator of this sphere, the area that we must cover grows larger, but once we pass the equator it begins to grow smaller. And so what might appear to us as slowing down and speeding up could just be the shape of space changing, and not the speed of expansion. Again, just food for thought.
  • brian-armstrong 8y ago
    Minor correction: The accelerating expansion of the universe implies the existence of dark energy, not dark matter.
    • InclinedPlane 8y ago
      This is not a minor correction simply because they both contain the word "dark". It would have been slightly more correct to confuse the terms dark matter and dark chocolate. Dark energy is an entirely different kettle of fish.
      • deleted 8y ago
        [deleted]
      • thx4allthestuff 8y ago
        Or maybe it's nothing at all ;)
    • thx4allthestuff 8y ago
      Correction noted and accepted. Thanks. In this particular thought experiment, dark energy (not dark matter as originally stated) has been replaced by the shape of space, and due to our inability to directly detect such extra-dimensional curvature we have defined this energy as a placeholder (which would not exist if this experiment proved to be true).
• GeekyBear 8y ago
  When your observations of the universe don't match up with what your theory predicts, it may just be possible that like Issac Newton, you just haven't gotten everything figured out yet. There may just be more going on in the universe over the largest scales than you've figured out so far, and we're waiting for an Einstein to come along to add to what we already know. Which is not to say that our inability to detect most of the matter in the universe means that dark matter is impossible. It just seems odd to write off the other possibility given the history of science.
• posterboy 8y ago
  > And that’s okay! Unless dark matter happens to be of a certain mass with a certain interaction cross-section, none of the designed experiments are going to see it. That doesn’t mean dark matter isn’t real, it just means that dark matter is something else than what our experiments are optimized to find. That's the rather underwhelming conclusion. And it's wrong insofar the size argument is not certain, but depends trivially on the distance of observation. Otherwise I'd like to know what magical number we are talking about.
• md224 8y ago
  This got me thinking about the difference between "direct" and "indirect" observation. What is the difference? For example: in these experiments, what would "direct" observation be? We have instruments that detect changes in certain variables, and we look for changes that align with our expectations of how a particle affects these variables. But we're not directly observing the particle... we're observing the particle's effects on these variables. So there's always an intermediary between us and the phenomena we're attempting to explore. It seems to me that this intermediary must exist for all phenomena that cannot be perceived by our 5 senses. So how do we determine when an intermediary is "direct" vs "indirect"? Looks like it's time for me to revisit Philosophy of Science...
  • ianai 8y ago
    Indeed, modern science is based on lots of prior knowledge. I like to remember that we could still be very wrong about how everything works. That’s what the missing 5/6ths of unexplainable behavior tells us, for one. For two, I hope (personally) that we are wrong about many things. Our current theories lead to some very bleak realities. But, ultimately, it’s important to not become skeptical to the point of tearing down progress. It’s one thing to keep an open mind and consider other options. It’s all too easy for bad actors to sell snake oil as a product of the unknown or unexplainable - or even just attack and destroy progress.
• wwarner 8y ago
  I learned a lot from this post. I didn't know that so many observations resulted in the same 5/6 ratio.
• chmaynard 8y ago
  I can say with confidence that there is no such thing as "dark matter", except as a metaphor. All we have is a theory about why a mathematical model doesn't quite fit the available data. Astronomers are masters at marketing because it keeps the money coming their way.
• golemotron 8y ago
  > But that’s indirect; we know there’s supposed to be a particle associated with it, and that’s what the hunt is all about. I think is the is the most important line in the article. Why does it have to be a particle? That's a large assumption. We've conceptualized everything in our models so far as waves/particles. Maybe we need a different concept.
• WhitneyLand 8y ago
  I see a lot of comments on mass, but I thought not too long ago there wasn’t even consensus this mass existed. For example the guy who tried to modify or extend gravitational theories to account for it. Maybe this was a fringe idea or has lost momentum.
• ridgeguy 8y ago
  I have no chops whatever in this field, but it's fun to think of experiments we might do: If I understand current theory, dark matter only interacts with itself and with ordinary matter through gravity. Hence the importance of Vera Rubin's observations [1] that some galaxies were rotating too darned fast to hold together based on the ordinary matter we could see. Gotta be something invisible generating more gravitational force. If so, couldn't we expect larger masses to attract more dark matter than lesser masses? And mightn't very high resolution measurements of those masses' gravitational forces disclose a discrepancy attributable to more dark matter clustering around a larger mass? I think of constructing two spheres, one of lithium (density = 0.534 g/cm^3), the other of platinum (density = 21.45 g/cm^3). Both have equal diameters, and very different masses. Park them out in space - maybe in an orbit inclined 90° to the ecliptic so there's some time when they're far away from the complicating effects of planetary masses. Then release test objects with accurately known masses (think the silicon spheres made for Gravity B Probe's gyroscopes [2]) near each of the two spheres. Minimum approach speeds would be given by the assumption of only ordinary matter in the spheres, no dark matter present. If there is dark matter, and if it accumulates according to gravitational interactions, the test mass approach speeds should be greater than calculated from ordinary matter gravitational force. Also, the larger mass should attract more dark matter and exhibit a greater deviation from ordinary matter force. Depending on the local density of dark matter, one might see the results change over time with differential accumulation of dark matter around the two spheres. Do we have the measurement capabilities to do something like this? I recall LIGO measures distances four orders of magnitude less than the width of a proton. [1] https://en.wikipedia.org/wiki/Vera_Rubin https://en.wikipedia.org/wiki/Vera_Rubin [2] https://einstein.stanford.edu/TECH/technology1.html https://einstein.stanford.edu/TECH/technology1.html
  • InclinedPlane 8y ago
    Your experiment has several fundamental flaws. Firstly, dark matter is weakly interacting (even with itself) which is why it doesn't "clump" the way atomic matter does. The only "clumping" of dark matter happens on galactic scales, because the typical speeds of dark matter particles are in the range of orbital speeds around galaxies. Our measurements of the dark matter mass in galaxies don't have nearly enough precision to be able to determine the relationship you posit with much certainty. As to your own experimental design, using large spheres of matter, it's completely non-workable. As mentioned above, dark matter particles have velocities in the range of orbital speeds around the galaxy, which is 100s of km/s. So most of the dark matter particles in the vicinity of Earth are going to simply pass through your spheres without stopping, and certainly without increasing their mass. Additionally, you seem to be confused about the scales of densities here. Dark matter is distributed in a somewhat uniform density on interstellar scales (there are density gradients across the galaxy however). Near the Sun the density of dark matter is about 0.0025 solar masses per cubic light-year, or about 5e-20 kg per cubic meter. So, there really is not a lot of dark matter passing through objects around Earth, mass wise. There's only maybe 6 kg within the entire volume of the Earth at any given time, for example.
• sanmon3186 8y ago
  Slightly off topic, but is there a resource for slightly less scientific minds to understand the universe as seen by those who do. I get an impression reading here and there that are theories to describe start and end of the universe (Or the limitations of human mind to grasp the concept of it), but I give up the moment I see too much of mathematics.
  • gauravphoenix 8y ago
    I highly recommend reading the book: we know no idea.
    • happy-go-lucky 8y ago
      You mean We Have No Idea: A Guide to the Unknown Universe?
      • gauravphoenix 8y ago
        Yeah
  • prewett 8y ago
    Hawking's "A Brief History of Time" is really good, and only has one equation. My mom read it, even.
• rurban 8y ago
  There still trying to hold up with such a silly idea, which is not supported by the standard model, measurements and not common sense. Time to admit that MOND is the better theory.