He merely saw the neutrino as a bookkeeping device to balance an equation about a neutron decaying into a proton and electron. Neutrinos weren't discovered in the lab until 1956. Nature creates neutrinos in at least three ways. For example, they are formed by nuclear explosions taking place in the center of stars. And staggering numbers of neutrinos were apparently a byproduct of the Big Bang, the explosion that may have spawned the universe.
Neutrinos also result from collisions between cosmic rays and Earth's atmosphere.
Detecting these miniscule particles is difficult, because they rarely interact with other particles. To spot enough interactions to make meaningful measurements, data must be gathered over along period of time using detectors that present the biggest target. In this case, the target was 12.5 million gallons of purified water in a cave lined with stainless steel. Detectors recorded the brief bursts of light given off by the rare interactions. The Japanese-US neutrino observatory, Super Kamiokande, also spotted the discrepancy.