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There's one thing I don't understand about the mystery of dark matter. I don't understand why the simple explanation for it isn't just that it's regular matter

There's one thing I don't understand about the mystery of dark matter. I don't understand why the simple explanation for it isn't just that it's regular matter that is not stars. Maybe there are just bajillions of planets and dust clouds out there. Matter that isn't directly circling stars, thus not reflecting light. Why is the popular assumption that if the mass isn't stars or things in orbit of stars, that it must be a mystery substance? I assume there's scientific reasoning behind this, but I've never heard it explained before. If someone could fill me in, that would be awesome.

lwat 14y ago
We know this is not the case because intersteller dust absorbs light, while the dark matter does not emit or absorb light at all. The only effect we see is the gravity.
- lloeki 14y ago
  Not only just light (photons) but no other radiation as well (electromagnetic, like X or gamma rays), except gravity.
  - ordinary 14y ago
    Gamma rays and X-rays are photons.[0] _____ [0] http://en.wikipedia.org/wiki/File:EM_Spectrum_Properties_edit.svg http://en.wikipedia.org/wiki/File:EM_Spectrum_Properties_edi...
derleth 14y ago
Dust absorbs light, blocking it (creating dark spots) and getting hot (creating light spots in a different frequency range). We know what interstellar dust looks like. Dark matter doesn't look like that; it doesn't look like anything. We only know about it because its gravity affects the things we can see. NASA has a brief page on dark matter and something else we know little about, dark energy: http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/ http://science.nasa.gov/astrophysics/focus-areas/what-is-dar...
InclinedPlane 14y ago
Science! We can tell how much dark matter is out there because we can "weigh" it through indirect measures. And then we can take different theories of dark matter (such as, say, the theory that it's all just a bunch of interstellar orphaned planets and "black dwarfs" and what-have-you made up of ordinary matter) and figure out what sorts of implications that would have, make predictions on observable effects of those different models and then test those predictions. And that is precisely what happened about 20-30 years ago. A lot of work was done to pin down what type of dark matter makes up the majority of it out there. For example, you can point a telescope at a set of neighboring galaxies and look for brightening effects due to gravitational micro-lensing from a chance alignment of a "macho" (e.g. orphaned gas giant planet) along the line of sight. Surveys were set up and indeed found that there were orphaned "macho" objects in our galaxy, but the statistics showed that they were orders of magnitude too rare to make up the bulk of dark matter we know about from other studies. Another line of evidence involves studying the large-scale structure of the Universe (e.g. the layout of galaxies, galaxy clusters, etc.) and comparing it with various computer simulations of models with different assumptions on the composition of the mass of the Universe (e.g. 100% "ordinary" baryonic matter, various percentages of "special" dark matter such as cold and hot dark matter, WIMPs, etc.) From this and many other lines of evidence we came up with very strong evidence that the vast majority of the mass budget of the Universe is in the form of so-called "cold dark matter" which is composed of weekly interacting massive particles other than neutrinos (neutrinos are dark matter, but we've been able to place an upper limit on how much they contribute to the dark matter budget of the Universe, because they are detectable to a degree, and it's only a fraction). So that's it, just a simple matter of comparing the predictions of different theories with observations and eliminating the theories that do not predict what we actually see out there in the Universe.
- mkup 14y ago
  (A bit off-topic question to physics expert on HN) I've read that supermassive black hole accretion is the most energy-effective process of mass to energy conversion in the Universe (50% efficiency or so). I'm just curious: Where does all that energy go? Extremely powerful jets of radiation are emitted into the intergalaxy space and then what? Does it just disappear? Isn't this energy responsible for Universe expansion? It must push galaxies away from each other, right?
JonnieCache 14y ago
On top of the stuff other people have posted, we can (apparently) peer at the cosmic microwave background and look for patterns in its anisotropy power spectrum from baryonic acoustic oscillations in the early universe's quark-gluon plasma. The results we see demand WIMPs. I'm not going to explain what any of that means because A. I've only recently woken up, B. you're perfectly capable of googling for yourself, and C. I'd probably get it wrong anyway. Suffice to say it's all brain-meltingly interesting. The excellent sean carroll has done a lecture on this: http://online.kitp.ucsb.edu/online/lens06/carroll/ http://online.kitp.ucsb.edu/online/lens06/carroll/ He has also written several long-ass blogposts: http://blogs.discovermagazine.com/cosmicvariance/2011/02/26/dark-matter-just-fine-thanks/ http://blogs.discovermagazine.com/cosmicvariance/2011/02/26/... http://blogs.discovermagazine.com/cosmicvariance/2012/05/09/dark-matter-vs-modified-gravity-a-trialogue/ http://blogs.discovermagazine.com/cosmicvariance/2012/05/09/... EDIT: you may also want to look up the Bullet Cluster. Or just read those blogs, it's all in there.

lwat 14y ago

We know this is not the case because intersteller dust absorbs light, while the dark matter does not emit or absorb light at all. The only effect we see is the gravity.

lloeki 14y ago
Not only just light (photons) but no other radiation as well (electromagnetic, like X or gamma rays), except gravity.
- ordinary 14y ago
  Gamma rays and X-rays are photons.[0] _____ [0] http://en.wikipedia.org/wiki/File:EM_Spectrum_Properties_edit.svg http://en.wikipedia.org/wiki/File:EM_Spectrum_Properties_edi...

derleth 14y ago

Dust absorbs light, blocking it (creating dark spots) and getting hot (creating light spots in a different frequency range). We know what interstellar dust looks like. Dark matter doesn't look like that; it doesn't look like anything. We only know about it because its gravity affects the things we can see. NASA has a brief page on dark matter and something else we know little about, dark energy: http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/ http://science.nasa.gov/astrophysics/focus-areas/what-is-dar...

InclinedPlane 14y ago

Science! We can tell how much dark matter is out there because we can "weigh" it through indirect measures. And then we can take different theories of dark matter (such as, say, the theory that it's all just a bunch of interstellar orphaned planets and "black dwarfs" and what-have-you made up of ordinary matter) and figure out what sorts of implications that would have, make predictions on observable effects of those different models and then test those predictions. And that is precisely what happened about 20-30 years ago. A lot of work was done to pin down what type of dark matter makes up the majority of it out there. For example, you can point a telescope at a set of neighboring galaxies and look for brightening effects due to gravitational micro-lensing from a chance alignment of a "macho" (e.g. orphaned gas giant planet) along the line of sight. Surveys were set up and indeed found that there were orphaned "macho" objects in our galaxy, but the statistics showed that they were orders of magnitude too rare to make up the bulk of dark matter we know about from other studies. Another line of evidence involves studying the large-scale structure of the Universe (e.g. the layout of galaxies, galaxy clusters, etc.) and comparing it with various computer simulations of models with different assumptions on the composition of the mass of the Universe (e.g. 100% "ordinary" baryonic matter, various percentages of "special" dark matter such as cold and hot dark matter, WIMPs, etc.) From this and many other lines of evidence we came up with very strong evidence that the vast majority of the mass budget of the Universe is in the form of so-called "cold dark matter" which is composed of weekly interacting massive particles other than neutrinos (neutrinos are dark matter, but we've been able to place an upper limit on how much they contribute to the dark matter budget of the Universe, because they are detectable to a degree, and it's only a fraction). So that's it, just a simple matter of comparing the predictions of different theories with observations and eliminating the theories that do not predict what we actually see out there in the Universe.

mkup 14y ago
(A bit off-topic question to physics expert on HN) I've read that supermassive black hole accretion is the most energy-effective process of mass to energy conversion in the Universe (50% efficiency or so). I'm just curious: Where does all that energy go? Extremely powerful jets of radiation are emitted into the intergalaxy space and then what? Does it just disappear? Isn't this energy responsible for Universe expansion? It must push galaxies away from each other, right?

JonnieCache 14y ago

On top of the stuff other people have posted, we can (apparently) peer at the cosmic microwave background and look for patterns in its anisotropy power spectrum from baryonic acoustic oscillations in the early universe's quark-gluon plasma. The results we see demand WIMPs. I'm not going to explain what any of that means because A. I've only recently woken up, B. you're perfectly capable of googling for yourself, and C. I'd probably get it wrong anyway. Suffice to say it's all brain-meltingly interesting. The excellent sean carroll has done a lecture on this: http://online.kitp.ucsb.edu/online/lens06/carroll/ http://online.kitp.ucsb.edu/online/lens06/carroll/ He has also written several long-ass blogposts: http://blogs.discovermagazine.com/cosmicvariance/2011/02/26/dark-matter-just-fine-thanks/ http://blogs.discovermagazine.com/cosmicvariance/2011/02/26/... http://blogs.discovermagazine.com/cosmicvariance/2012/05/09/dark-matter-vs-modified-gravity-a-trialogue/ http://blogs.discovermagazine.com/cosmicvariance/2012/05/09/... EDIT: you may also want to look up the Bullet Cluster. Or just read those blogs, it's all in there.