There has been no notable evidence for the existence of such radiation; baryonic matter contains multiple interacting particle types, but it is not known if dark matter does. Cosmic microwave background data may indicate that the number of effective neutrino degrees of freedom is more than 3.046, which is slightly more than the standard case for 3 types of neutrino. 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 the sterile neutrino.