Over time, traces from those neutrinos that had been created in the atmosphere started to reveal a pattern. Those arriving from above came in the expected proportion and number. But too few muon-neutrinos arrived from below. Two large groups of physicists worked independently to explain why. Each neutrino is in fact a mixture of three mass states.
Theorists figure that there is no way to have mass states without having mass.
Many lines of evidence suggest that there is about 10 times more matter in the universe than human instruments can see. Neutrinos will now fill in some of that missing matter. Whether neutrinos weigh enough to make a significant difference in the fate of the universe and the composition of matter remains a mystery. We're still far from that. But there are some very important experiments in the wings that may answer those questions.