From PBS Digital Studios’ The Good Stuff: Dark matter makes up 70 percent of the known universe, and we know very little about it. This video on dark matter includes an interview with theorist Patrick Fox and a tour of the MINOS underground area at Fermilab.
If neutrinos were massless, they would be always traveling at the speed light. This means that, as Einstein informed us, their clocks would not tick, and thus neutrinos could not change from one type to another. This stands in contradiction to neutrino oscillations observed in many neutrino experiments, such as the transformation of a muon neutrino into an electron neutrino recently reported by the Fermilab NOνA experiment.