Neutrino

A proposed particle was suggested to solve the problem of beta-particle emission. This particle, named the neutrino, was theorized to be neutral and have very little or no mass. It was very difficult to detect and remained backed by no observational evidence for many decades.
 Wolfgang Ernst Pauli Austria 1931 Neutrino

A proposed particle was suggested to solve the problem of beta-particle emission. This particle, named the neutrino, was theorized to be neutral and have very little or no mass. It was very difficult to detect and remained backed by no observational evidence for many decades.