Different Cosmologies for dark matter, or dark radiation, have different observational prospects. Standard Model particles were in thermal equilibrium at early times. But maybe the dark sector was not. In this talk I will discuss a few scenarios where particles are produced by non-equilibrium processes in the dark sector. I will show how dark matter, such as sterile neutrinos, could have been produced through an explosive period of exponential growth (similar to the spreading of a pathogen). I will also describe how radio photons can be produced by resonant oscillations of dark photons, as a possible explanation of the Arcade radio excess.