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In the thermal dark matter (DM) scenario, it is the interaction of the DM particles with the Standard Model (SM) particles that kept the DM particles in kinetic equilibrium with the thermal plasma.

Why is it that today the DM particles do not have appreciable non-gravitational interaction with the SM particles like they had in the early Universe? Do I misunderstand something?

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    $\begingroup$ Reaction cross-sections (i.e. probabilities) can be energy-dependent. This is the case here. As the universe expanded, the average KE of dark matter decreased. $\endgroup$ – probably_someone Dec 18 '17 at 19:44
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Dark matter have clearly negligible cross-section with baryonic matter at current low temperatures, otherwise we would have detected it by now in experimental searches. But that cross-section can be higher at higher energies. In the early universe, there was enough pressure and temperature for this to happen often enough

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  • $\begingroup$ Can you please include the mathematical expression for the expected variation of cross-section with temperature? @lurscher $\endgroup$ – SRS Jun 14 '18 at 12:32
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Dark matter ($\psi$) typically is thought to interact with ordinary matter ($f$) via the reaction $\psi\psi\rightarrow f\bar{f}$ where the bar indicates the anti-particle. As two dark matter particles have to come together to form ordinary matter, this implies the rate of the reaction is proportional to the square of the dark matter density. In the early Universe the density was higher and so dark matter interactions were more frequent.

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  • $\begingroup$ In addition to the interaction $\psi\psi\to ff$, there were also scatterings of the form $f\psi\leftrightarrow f\psi$... $\endgroup$ – SRS Dec 19 '17 at 9:24
  • $\begingroup$ True, but as everything is in thermal equilibrium in the early Universe, we will still have the density squared dependence. $\endgroup$ – Virgo Dec 20 '17 at 1:09

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