>>938570481
let's take a look at beta(-) decay. a neutron pops into a proton and an electron. now check the mass of a neutron, proton and electron. you'll see, that the masses don't add up. the neutron has more mass than protons and electrons combined.
by law of thermodynamics energy can only be transformed. mass is a form of energy.
what is easily observable are particles like neutrons, protons and electrons.
now imagine: you pop a neutron, you detect a spare proton and a spare electron. but nothing else. but the masses don't add up. well there has to be something that you can't detect. lets call this a neutrino. boom, now the equation is integer: mp + me + mv = mn.
you assume the neutrino is there even you never detected it.