https://www.scientificamerican.com/article/mysterious-neutrinos-get-new-mass-estimate/

Neutrinos, some of nature’s weirdest fundamental particles, are nearly massless—emphasis on nearly. They were predicted to be completely massless, but experiments roughly 20 years ago found they surprisingly do have some mass. Just how much has remained a mystery. Now a new calculation based on cosmological observations places an upper limit on how heavy the lightest kind of neutrino can be.

There are many strange things about neutrinos: their unexpected heft, for one thing, and that they rarely interact with other matter and are passing through our bodies by the billions each moment. Perhaps the oddest aspect of these particles is their tendency to switch identities, cycling between the three possible “flavors,” or types. In fact, it was the observation of this shape-shifting ability in the first place that told scientists the three neutrino flavors must have different masses—which means, of course, that all of their masses cannot be zero.