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?
Answer
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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