Making dark matter out of light: the cosmology of sub-MeV freeze-in

Dark matter could be a “thermal-ish" relic of freeze-in, where the dark matter is produced by extremely feeble interactions with Standard Model particles dominantly at low temperatures. In this talk, I will discuss how sub-MeV dark matter can be made through freeze-in, accounting for a dominant new channel where the dark matter gets produced by the decay of plasmons (photons that pick up an in-medium mass in the primordial plasma of our Universe). I will also explain how the resulting non-thermal dark matter velocity distribution can impact cosmological observables, including the CMB, the Lyman-alpha forest, and the cosmological 21 cm signal.