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TLDR: Not as such, but it does give us some constraints on its properties. Background: A galaxy cluster consists of three major components, from smallest to largest proportion of mass: galaxies (visible with optical telescopes), intracluster gas (visible with X-ray and radio telescopes) and dark matter (not directly observable). Dark matter makes up about 80%...

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Disclaimer: I'm Dr. Kevin Croker, lead author on the ApJ series in question. I work on formal aspects of relativistic perturbation theory. I think the best way to answer your question is to just address all of the commenters' responses. I just made this SE account to respond to you, so I don't have sufficient reputation to reply as a comment yet. (For ...

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The luminosity of normal stars is a strongly increasing function of mass. e.g. $L \propto M^3$. If another star is "hidden" in a binary system, then it is of lower mass. So the amount of hidden mass is less than what is seen. Of course this can be accounted for when estimating the mass present in luminous matter because we know typical binary ...

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Yes. Quoting values from wikipedia (which in turn cites Ade et.al. in Astronomy and Astrophyics 517), the contribution of matter (both Dark and visible matter) is $$Ω_\text{mass} ≈ 0.315±0.018$$ The contribution of photons and neutrinos is small, and within the boundaries of error of the other terms: $$Ω_\text{relativistic} ≈ 9.24×10^{−5}$$ And the ...

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One explanation I found helpful went as follows: consider a volume of space which expands with the universe by a factor of $a$. The radiation density in it varies by a factor of $a^{-4}$ ($a^{-3}$ because the photons get more spread out, and a further $a^{-1}$ due to the cosmological redshift (each photon becomes less energetic)). The matter (dark and ...

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