First observed directly by the Cowan-Reines neutrino experiment.
Because they interact weakly with matter and mostly just escape out of nuclear reactors, you can likely locate all nuclear reactors on Earth by measuring neutrino flows:The CIA must love that shit, they must have had it years prior to this public paper.
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A neutrino is a subatomic particle that is part of the Standard Model of particle physics. It is electrically neutral, has an incredibly small mass (which is still not precisely measured but is known to be much less than that of an electron), and interacts very weakly with other matter, meaning it can pass through ordinary matter almost undetected.