  It is entirely possible that our Universe is filled with dark radiation, such
as SM neutrinos or new physics states, that are sourced by the decay of dark
matter with cosmologically long lifetime. If non-thermal neutrinos produced
such way carry sufficient energy, they can leave a detectable imprint in
experiments designed to search for very weakly interacting particles: dark
matter and underground neutrino experiments. Conversely, a very soft
non-thermal population of dark photons sourced this way is allowed to exceed
the number density of CMB photons by many orders of magnitude without being in
conflict with current bounds. Equipped with a small probability of conversion
into ordinary photons, the scenario becomes testable through the cosmological
21cm signal.
