# Why did dark matter interact with the Standard model particles in the early Universe but not now?

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?

• 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. – probably_someone Dec 18 '17 at 19:44

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.
• In addition to the interaction $\psi\psi\to ff$, there were also scatterings of the form $f\psi\leftrightarrow f\psi$... – SRS Dec 19 '17 at 9:24