The neutrino itself was postulated in just this way, to balance mass-energy equations of beta decay. It looked like energy was being lost, but if some was being

The neutrino itself was postulated in just this way, to balance mass-energy equations of beta decay. It looked like energy was being lost, but if some was being carried away by neutrinos, problem solved. The alternative was to reformulate the conservation laws. Turns out it was neutrinos.