What is a neutrino?

A neutrino is a fundamental particle of matter, meaning that it has no substructure (it can't be broken down further into components). It has a very low mass, so the gravitational force exerted on it and by it is negligible. It has been described as "ghostly" because it usually travels through objects without interacting with them in any observable way. The Sun produces about 2*10³⁸ neutrinos per second through fusion reactions. It has also been observed that there are three different types, or "flavours" of neutrinos, each with a different mass. There are electron neutrinos, muon neutrinos, and tau neutrinos. Neutrinos used to be thought of as massless, but it turns out their mass is just extremely small. Neutrinos (or antineutrinos) are produced by beta-minus or beta-plus decay, which is mediated by what is known as the "weak" force.