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In my readings of the DAMA/Libra dark matter detector articles, it is said to detect a dark matter particle by seeing light emissions from a sodium iodide crystal ion collision. My question is why would the collision happen if dark matter doesn't interact with ordinary matter?

icedcoffee
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    Sodium iodide is used because it scintillates (emits light) in response to ionization. The assumption is that rare interactions of dark matter with baryonic matter produce ionization. – John Doty Jun 03 '22 at 18:58

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We know that dark matter does interact with ordinary matter through gravity. But gravity is very weak. So, indeed you are right: If dark matter /only/ interacts with ordinary matter through gravity, there is no reason to expect it to collide with the sodium iodine crystal.

Now, should we expect dark matter to interact with us in some other way? Interestingly, the average cosmological densities of dark matter ($\Omega_{darkmatter}$) and luminous matter ($\Omega_{baryons}$) are somewhat similar, $\Omega_{darkmatter}\sim 5\Omega_{baryons}$. That would be unexpected if gravity was the only connection between the two. Further, observations of scaling laws in galaxies also point towards a connection between the two that is much stronger than what one would expect from gravity alone. So, I think it is fair to say that yes, we should expect there to be interactions between dark matter and us other than just through gravity.

That still doesn't mean that dark matter, when it interacts with a sodium iodine crystal, would be visible in that experiment, given its detection principle, energy threshold, etc. For that to be your expectation, you need to assume that dark matter is in the mass range typical of other particles and atoms (i.e. proton mass up to 1000s of proton masses), and that it has a sufficiently high scatter probability, like that from Z-mediation at loop level or through Higgs mediation. Dark matter particles that have these properties are called WIMPs; of course, many other dark matter models have been proposed.

rfl
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    Can you further explain when you say that the cosmological density of dark matter and baryons is somewhat similar? Protons and neutrons are concentrated in planets and stars, while dark matter is spread over a vast halo surrounding galaxies,. – foolishmuse Jun 03 '22 at 17:25
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    Hey @foolishmuse, I clarified that this concerns the average cosmological densities, i.e. averaged over some megaparsec length scales. Please realize that Earth is a incredibly special and unusual place in the universe: The average baryonic density of the universe is some $4\times 10^{-31}g/cm^3$, whereas a piece of rock has g/cm$^3$, i.e. 30 orders of magnitude more. – rfl Jun 03 '22 at 18:34
  • Thanks. Sort of like saying the average net worth between me and Bill Gates is $45 billion. – foolishmuse Jun 03 '22 at 18:56
  • LOL. Yes, exactly like using average income when comparing countries, etc, instead of the median! – rfl Jun 03 '22 at 20:16
  • Ok. When you say the "average cosmological density of dark matter," is this being calculated solely by the level of gravity that the DM generates? Or some other method? – foolishmuse Jun 03 '22 at 21:32
  • There areany different pieces of evidence for darkatter. But yes, they are all gravitational only. – rfl Jun 04 '22 at 06:07
  • Goes to the theory that there is not any actual matter at all, only the solar wind of charged particles from trillions of stars causing an excitation and polarization of the gluon field leading to time dilation and gravity – foolishmuse Jun 04 '22 at 11:22
  • Hahahahaha, LOL – rfl Jun 04 '22 at 15:30
  • Better options? – foolishmuse Jun 05 '22 at 12:31