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One thing I don't understand and maybe someone can take the time to explain. Why does it have to be this "Dark Matter" causing the mass differences we've been

• burkaman 10y ago
  I think dark matter is a synonym for "undetectable-through-normal-means matter". It's just a description of something mysterious causing effects that some kind of matter would cause. It could be a mix of different kinds of undetectable matter, or some fundamental flaw in our understanding of matter and gravity, or something else. The arguments against aliens and mistakes are presumably that the effects appear consistently across the whole visible universe, and through many different measurements.
• krastanov 10y ago
  "undetectable-through-normal-means matter" is by definition not normal matter. We do not know what it is, so we have a placeholder name for it: "dark matter", i.e. matter that does not shine in the EM spectrum. We have theories about what it is, but most of the mundane ideas (like cold dust) have been ruled out by observations. Now we are left with all these stranger ideas. "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."
  • adwn 10y ago
    > "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth." Unless you made a mistake in your elimination, which is sometimes more probable than the apparently single remaining truth. Not saying this is the case here, just that this basic principle should be used with great care.
• empath75 10y ago
  > undetectable-through-normal-means normal matter I assume by this you mean dust and planets and such. https://medium.com/starts-with-a-bang/could-dark-matter-just-be-normal-stuff-thats-dark-5e680e7a066e#.xnhcgnbfi https://medium.com/starts-with-a-bang/could-dark-matter-just... That also goes for 'aliens', because presumably, they'd also be made of normal matter, and if they're not, then we're back to some exotic form of matter like dark matter. As far as being a mistake, it could be possible that we are fundamentally misunderstanding something about how gravity works. That would be as interesting a result as dark matter, to be honest.
• Florin_Andrei 10y ago
  The only thing we're sure it exists is - there's something out there that acts exactly like normal matter with mass, and it interacts gravitationally with itself and with regular matter, but it doesn't seem to interact in any other way. That's it. The name "dark matter" is a bit optimistic, because we don't even know if it's anything like "matter". It's not a fluke in the equations, it's not a fifth fundamental force. All these alternatives have been shown to be very unlikely. It's not "normal" matter because that one would interact in ways other than gravity, too, and it would tend to make "clumps" (stars, planets). This one doesn't make clumps. So, yes, it's pretty certain that it exists, but nobody knows what it is. Its only attribute observed so far is its mass. Maybe we should call it "dark mass" instead.
  • auvrw 10y ago
    upvoted, and not disagreeing (b/c i know nothing a/b this) -- would like a similarly semisimple explain on how "interaction" thru gravity alone makes any sense at all in the relativity framework: light no longer travels along lines of zero acceleration? Edit: on second thought, a book reference is probably better
    • mikeash 10y ago
      In relativity, gravity isn't a force that causes acceleration, it's a change in the shape of space which changes the path objects take. Thus, gravity affects the path light takes, but that's still a "straight line," because what gravity is really affecting is what a straight line is.
      • effingwewt 10y ago
        This is my favorite explanation of this, mind if I forward this to an old Astronomy Professor? I think he'd love this too.
        • mikeash 10y ago
          Sure! Just beware that I'm far from an expert and may well have gotten something hilariously wrong. If he rips it to shreds, please let me know what he had to say.
• semaphoreP 10y ago
  Basically, dark matter is something that has gravitational pull, but shows no other evidence of its existence. We have seen evidence for dark matter in galaxy rotation curves, galaxy cluster interactions, and the cosmic microwave background fluctuations, so we know it's not a mistake (each of these measurements are independent and do not share common systematic errors). We know it's not normal matter (e.g. black holes), because we at least somewhat understand the physics of how normal matter works, and we would expect to see something observationally if it is just normal matter. Since it is not normal matter, using Occam's razor, we can only postulate it is something that has gravitational pull but doesn't interact otherwise. Perhaps once we figure out how to study dark matter, we can tease out some of its properties and it won't just be all lumped together as "dark matter".
  • netcraft 10y ago
    There was a recent idea that small, primordial black holes might be related to dark matter http://www.nasa.gov/feature/goddard/2016/nasa-scientist-suggests-possible-link-between-primordial-black-holes-and-dark-matter http://www.nasa.gov/feature/goddard/2016/nasa-scientist-sugg... Basically the idea that theyre smaller and more common than previously expected - it would be really hard to observe them if they weren't in-between us and something else or snacking on something.
• symmetricsaurus 10y ago
  Dark matter can explain: the large scale evolution of the universe (together with dark energy but whatever), rotational velocities of galaxies, gravitational lensing and other phenomena. No other theory explains all of these things as well or simply. Also, dark matter is not really so exotic and weird. We already know of one type of dark matter particle, neutrinos. To think that there are other more massive particles that only interact through gravity and the weak force isn't much of a strectch in that light.
  • gpderetta 10y ago
    Exactly, in fact the hypothetical massive sterile neutrinos, which are part of one of the possible explanations for the neutrino masses, are one of the dark matter candidates.
• T0T0R0 10y ago
  Exoplanets are dark matter. They do not emit light, so they are dark. You can't see an exoplanet, you have to look for stars that wobble. Asteroid belts are also dark matter. If a remote star has an asteroid belt, it wouldn't be easily detectable, since it probably wouldn't elicit the kepler-style wobbles and flickers, or if it did, perhaps less obvious/detectable events. Based on this, the astronomical fad terminology is flawed, since it seems to claim that only stars matter because only stars are matter (and the luminous gas of nebulae too, of course). The layperson finds intrigue in the term Dark Matter, because journalists are trying to sell a story.
  • netcraft 10y ago
    im pretty sure this isnt true - if any of those things were massive enough to substitute for the gravitational effects that have been observed they would emit infrared radiation at least.
    • T0T0R0 10y ago
      So go ahead and observe this all-too-obvious infrared, that exoplanets must surely emit. Meanwhile, their gravity is now well known to induce wobble on their parent stars, which are much more luminous, and probably outshines any exoplanet in the infrared.
      • GFK_of_xmaspast 10y ago
        On the other hand, this has already been done: https://en.wikipedia.org/wiki/List_of_directly_imaged_exoplanets https://en.wikipedia.org/wiki/List_of_directly_imaged_exopla...
        • T0T0R0 10y ago
          This method works best for young planets *that emit infrared light* and are far from the glare of the star. In other words, if they are still swirling balls of liquid magma. So, NOT dark matter.
• mikeash 10y ago
  "Dark matter" just means "stuff we can't detect except by gravity" so that covers your first case. There's a theory that "dark matter" isn't matter at all, but some modification to the currently accepted laws of physics. Wikipedia lists some of these ideas: https://en.wikipedia.org/wiki/Dark_matter#Alternative_theories https://en.wikipedia.org/wiki/Dark_matter#Alternative_theori... Ultimately, it's generally assumed to be some sort of actual matter because that's what best fits the observations. No alternative explanation has been raised that better fits the observations than the explanation that it's some unknown form of matter that doesn't interact with electromagnetic radiation very much, but still participates in gravity. As for what those observations are, galaxies are observed to rotate much differently than if they only had the mass that was visible, galaxies cluster as if there was more mass than can be seen, gravitational lensing indicates that galaxies mass more than what's observed with light, the cosmic microwave background spectrum is different from what you'd expect from only the stuff we can see, and a fair bit more mentioned on that Wikipedia page. Keep in mind that any alternative explanation needs to not only explain the fact that these things are different, but also the exact quantity by which they are different. So far, "dark matter" is the best explanation anyone's come up with.
  • orf 10y ago
    > some unknown form of batter Mmhhhh... I'm pretty hungry right now and that gave me some wonderful imagery.
    • mikeash 10y ago
      Oopsie, stupid hands. Fixed it.
  • sandworm101 10y ago
    I would call 'dark matter' a description rather than an explanation. It seems to effect gravity. So it has mass. So it's 'matter'. But other than that it is unobserved, and so 'dark'. A+B = a very short but accurate description of an observation, rather than an explanation of why that observation happens.
• tomr_stargazer 10y ago
  (PhD student in astrophysics answering here.) This is a great question! You're addressing the idea (roughly along the lines of Occam's razor) that if there's any way to explain the "extra mass" without invoking a new form of matter, it should be preferred. Most of these other possibilities have been largely ruled out via careful observations, which are detailed here: https://en.wikipedia.org/wiki/Dark_matter#Composition https://en.wikipedia.org/wiki/Dark_matter#Composition One of these possibilities, for example, is that the missing matter is actually contained in a great many small, dark, massive objects scattered throughout galaxies -- such as failed stars, planets, or even black holes -- rather than in a diffuse, invisible material. This possibility has actually been largely ruled out through a number of statistical "microlensing" surveys that are sensitive specifically to the presence of massive, dark bodies (via their gravitational lensing of background stars -- a rare event, but measurable statistically). https://en.wikipedia.org/wiki/Massive_compact_halo_object#Detection https://en.wikipedia.org/wiki/Massive_compact_halo_object#De... I think the wikipedia Dark Matter article is actually super well written and should address these issues, too!
  • semi-extrinsic 10y ago
    Regarding the "no massive dark bodies" result: does this also cover dark dust? Or is that otherwise ruled out (perhaps by optical density)?
    • maverick_iceman 10y ago
      Baryonic dust is ruled out by BBN.
    • okket 10y ago
      If you have four times of visible matter as "dark dust", this dust would clump together, creating stars, becoming visible matter again.
    • tomr_stargazer 10y ago
      Additional dust inside a given galaxy would both absorb optical light and emit infrared light (depending on its temperature, which is set by the Galaxy's ambient starlight among other factors); both effects are readily detectable.
• maverick_iceman 10y ago
  Big Bang Nucleosynthesis (BBN) says that the amount of baryonic (i.e. usual) matter cannot be much higher than what we directly observe. However, it accounts for only ~16% of the mass necessary to account for galactic rotation curves, gravitational lensing by galaxies etc. So the leftover must be non-baryonic dark matter. One can of course say that BBN is wrong but that's very unlikely as it has been tested to high precision from CMB study.
• grahamburger 10y ago
  > How do we know it isn't undetectable-through-normal-means normal matter As I understand this is kind of what we think it is, which is why it's called 'dark.'
• stouset 10y ago
  > Why does it have to be this "Dark Matter" causing the mass differences we've been seeing? How do we know it isn't undetectable-through-normal-means normal matter, or aliens, or mistakes in our calculations/observations, or anything else? "Dark matter" is just a name for "a phenomenon that only appears to interact with the rest of the universe through gravitational fields, and not through elecromagnetic fields, the weak, or the strong nuclear force". There are many competing theories for what dark matter actually "is". The name is independent of any particular theory that describes its true nature, whether that's an exotic form of matter, aliens, or mistakes in our observation.
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• SmellTheGlove 10y ago
  > undetectable-through-normal-means normal matter I'm not a physicist, but my understanding was that dark matter is undetectable-through-normal-means because it doesn't interact with normal matter. If it were undetectable-through-normal-means normal matter, it would be detectable through normal means.
• yincrash 10y ago
  Matt O'Dowd, an astrophysics professor, hosts a YouTube series called PBS Space Time that does a really effective job if explaining the evidence of dark matter in this episode: https://youtu.be/z3rgl-_a5C0 https://youtu.be/z3rgl-_a5C0