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Does dark-matter interact with gravity like light-matter does, or does dark matter interact only with light-matter via gravity?

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    $\begingroup$ The standard term would be "baryonic matter" rather than "light matter." $\endgroup$ – Ben Crowell Aug 25 '18 at 20:35
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    $\begingroup$ I can recommend as a min checking wikipedia or even Google. Come here if you get stuck $\endgroup$ – JMLCarter Aug 25 '18 at 22:24
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    $\begingroup$ @JMLCarter It's hard, because there are thousands of models of dark matter, and popsci resources like Wikipedia have a nasty habit of picking one and saying this is what dark matter is. Unfortunately the only honest answer to OP's question is, we have absolutely no idea. $\endgroup$ – knzhou Aug 26 '18 at 1:51
  • $\begingroup$ Also see physics.stackexchange.com/questions/52877/dark-matter-stars $\endgroup$ – Rob Jeffries Aug 26 '18 at 10:28
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The idea behind Dark Matter is that it interacts with 'regular'/baryonic matter via gravity; however, it does not seem to interact with electromagnetic radiation, which is why we can't see it (hence it is called 'dark').

It's existence has not been proven. It is thought that it may exist, because certain astrophysical phenomena that have been observed (e.g. galactic rotation curves) don't appear to be correct, unless there is a large amount of extra mass present, which is not accounted for by the matter we can 'see' (i.e. the matter that interacts with EM waves).

Because it interacts with ordinary matter via gravity, it is thought that large quantities of dark matter are 'clumping together' with and forming part of the galaxies we can see (including our own Milky Way). This is the whole point - because it would provide an explanation for the observed galactic rotation curves, as already mentioned.

It is possible that there could be dark matter 'galaxies', which contain no 'normal' matter. However, given that the dark matter doesn't interact with EM waves, it is hard to see how we could detect them (again, it is called 'dark' for a reason ..), except by gravitational lensing. It seems unlikely that it could form stars though, as that would presumably generate a lot of energy, which we should then be able to observe. This is also inconsistent with the idea that it only interacts via gravity (a dark matter black hole might be possible?). So, perhaps it is better to think of there possibly being 'clouds' of dark matter, where there is little or no 'normal' matter present.

I think the body of your question is a bit confusing. Both of those statements are true: dark matter interacts with gravity like 'light-matter' does and it only interacts with 'light-matter' via gravity.

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Does dark-matter attract itself with gravity as light-matter does, and if so where are the dark stars and galaxies?

The above question is different than the question in the body :

Does dark-matter interact with gravity like light-matter does, or does dark matter interact only with light-matter via gravity?

This last has been answered by Time4

The answer to the title is " we do not know", there are various hypotheseis that are being made, but little observational evidence to date. See this .

Within standard cosmology there is no reason why only gravitationally interacting particles would clump separately from baryonic matter in the large clusters. A new theoretical model would have to be proposed.

If for example one observed shadowing of galaxies by something "dark", as in the link for smaller clumps, one could start thinking on new lines.

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