In the original theory of the neutrino they were thought to be massless just like the photon. However, experimental evidence suggests that observed neutrinos o

In the original theory of the neutrino they were thought to be massless just like the photon. However, experimental evidence suggests that observed neutrinos oscillate between the three neutrino flavours over time & the sane way to allow for that theoretically is for them to to have slightly different small masses. The Wikipedia pages on the Neutrino & Neutrino Oscillation give a good overview.