Dark radiation

Dark radiation is a postulated species of radiation that mediates interactions in the dark sector. That is, just the way photons mediate electromagnetic interactions between particles in the Standard Model (baryonic matter in cosmology), dark radiation is supposed to mediate interactions between dark matter particles. And, similar to dark matter particles, dark radiation doesn't interact with Standard Model particles. Though there has been no notable evidence for the existence of such a species, since the baryonic sector contains multiple interacting species, it is reasonable that the dark sector also does. Moreover, it has been pointed out recently that the cosmic microwave background data seem to suggest that the number of effective neutrino degrees of freedom is more than 3.046, which is the standard case for 3 species of neutrinos. This extra degree of freedom could arise from having a non-trivial amount of dark radiation in the universe. One possible candidate for dark radiation is a sterile neutrino.