What if dark matter is just regular matter that's not very well lit up? What do you think?
No. Big Bang Nucleosynthesis (BBN) places very stringent limits on the amount of ordinary matter (baryons) in the universe. If all of the dark matter were made up of ordinary atoms, there would have been many more heavy nuclei created during the big bang than we see. The cosmic microwave background (CMB) radiation also places limits on the amount of baryonic matter in the universe. Bottom line - it is very clear that the dark matter is not made up of ordinary atoms.
The problem is that if dark matter is really just regular matter, it should be lit up. With all the EM radiation shooting through the cosmos we should be able to see it if it were just regular matter. But we can't see it. So something is up.
Define "regular matter".
Matter that isn't dark, of course!
There are a variety of ways that we measure the amount of dark matter that is out there. One is to use gravitational lensing to estimate the mass of an entire galaxy or galactic cluster, and then to estimate ordinary matter based upon the luminosity of the visible stars which astronomy observations lead us to infer constitute the vast majority of baryonic matter, and then to adjust slightly for non-luminous sources of ordinary matter like neutrinos, interstellar gas, planets, stellar black holes, MACHOs etc.
The disparity is too great to be accounted for with ordinary baryonic matter. It isn't implausible that are best estimate of ordinary matter is a bit low and misses some of the non-luminous part of ordinary matter. But, the amount of dark matter observed through gravitational lensing and the apparent spatial distribution of the dark matter rule out the possibility that is is mostly ordinary matter. Dark matter is arranged in space in a manner that is too clumpy to be neutrinos (a.k.a. "hot dark matter") and that is not clumpy enough to be ordinary baryonic matter.
Also, because we can estimate the mass of dark matter per volume in our part of the Milky Way to better than an order of magnitude based upon the dynamics of the Milky Way' visible matter, we know that if dark matter were ordinary matter that we could have detected it in direct dark matter detection experiments. But, the null detection in those experiments rules out that possibility.
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