What is the origin of neutrino masses?

Neutrinos oscillate between flavours, proving they have non-zero but tiny masses — at least a million times smaller than the electron's. The Standard Model, in its minimal form, predicts massless neutrinos.

Whether neutrinos are Dirac or Majorana particles, the absolute mass scale, the mass ordering, and the source of their smallness (see-saw mechanism?) are central open questions.