Question

How do we know that the dark matter is cold or non-relativistic?

Answer 1

Hey dear here is the answer


Dark matter is often assumed to be cold because if it were hot it would radiate, and hence be visible ('light' matter, perhaps?). Not all theories demand it be cold, some versions of WIMPs would not radiate simply because they do not interact with the electromagnetic force.

Non-relativistic (I assume you mean velocity << c?) comes from the apparent distribution of dark matter inside galaxies: For the matter to be where it needs to be to give the mass distribution inferred from the orbits of the visible matter, it must have a non-relativistic orbital velocity. Assuming it's orbiting of course, but if its not then we need a very exotic explanation of how it got there.

I don't know how you could ever totally rule out the possibility of dark matter - even if you could prove a theory that does not invoke dark matter to explain the various missing mass observations (MOND for example), you couldn't prove that there wasn't still a small amount of dark matter hidden in some corner of the universe




Hope its help you

Answer 2

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Well , Lets just say that when you look at the high red shift universe you're in a sense looking into the past.

Well aware with the universe at that redshift and knowing our own results us 2 points in between the interval between two events which show that the structure in the universe forms in a bottom up way.

$<b>Therefore , the dark matter should be cold.</b>$

#BeBrainly ❤️