The three known flavours of neutrinos are the electron, muon, and tau. Neutrinos oscillate among the flavours as they move. It is hard to determine an exact upper bound on the collective average mass of the three neutrinos. For example, if the average neutrino mass were over 50 eV /c (less than 10 of the mass of an electron), the universe would collapse. CMB data and other methods indicate that their average mass probably does not exceed 0.3 eV/c. Thus, observed neutrinos cannot explain dark matter.