Question

Do neutrinos have mass?​

Answer 1

For decades physicists thought neutrinos weighed nothing, and they were shocked in 1998 to discover that the particles do have very small, but nonzero, masses. Exactly how much mass they have is still unknown. The larger question, however, is why they have mass at all.

Answer 2

One thing that seems to be for sure: the neutrino can't weigh more than 50 electronvolts (one electronvolt is roughly 1.783×10−36 kg)

. It's a "Model", where initially neutrinos are considered massless, because no mass was observed. The way we know, now, that neutrinos have masses, is through the mixing between the different neutrino types, through a matrix called the PMNS matrix (similar to the CKM matrix for quarks