Abstract:  Dark matter could be a thermal relic of freeze-in, where the dark matter is produced by extremely feeble interactions with Standard Model particles dominantly at low temperatures. The simplest sub-MeV dark matter models with freeze-in include models with a kinetically-mixed dark photon mediator, or equivalently models where dark matter is millicharged under the Standard Model U(1). In this talk I will discuss how such models can impact and be constrained by cosmological observables, including the CMB, the Lyman-alpha forest, and the EDGES observation of the cosmological 21 cm global signal.