Neutrino

In 1930 the physicist, Wolfgang Pauli (1900-1958), postulated a new fundamental particle of the universe, subsequently (1935) named by the physicist, Enrico Fermi (1901-1954), the neutrino ("little neutral one" in Fermi's Italian), an electrically uncharged particle associated with the negatively electrically charged particle, the electron, but presumed to have no mass.

By the time Pauli's postulated the existence of the neutrino, physicists had already discovered that atoms, previously thought of as homogeneous and indivisible, consisted of sub-particles, called subatomic particles, specifically, protons and neutrons localized in a center-of-the-atom nucleus, the major owners of atoms' mass, and, by comparison tiny, electrons surrounding the nucleus. The protons each carried a unit of positive electrical charge and the electrons, equal in number to the number of protons, each carried a unit of negative electrical charge, rendering the atom as a whole electrically neutral, inasmuch as the neutron itself carried no electrical charge.

They had also discovered that the some atoms were unstable, in that they might emit one or more their subatomic particles, a process called radioactivity...

Basing his thinking based on the widely accepted law of conservation of energy, Pauli postulated the existence of an electron-associated presumed massless neutrino to reconcile the observed discrepancy between the energy carried by an electron emitted during the radioactive process called 'beta decay' by an atom's nucleus — electron energy too small — and the energy change of the nucleus, the missing energy carried off by the postulated chargeless neutrino particle.