What do they do? Are they just floating around?
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Neutrinos make up 3 of the 12 ‘matter’ particles (also called fermions). They are created in vast quantities by nuclear reactions such as the fusion in the sun. They are small… really small. You know how an electron is supposed to be small? Neutrinos are 500,000 times smaller. In fact, they were thought to have no mass or a while. And yes, they are just flying around everywhere. About 65 billion of them pass through every square centimeter every second. But they are so ridiculously tiny that they have almost no effect on anything.
You know how electrons are much smaller than protons and have a negative charge?
Neutrinos are even small than electrons (less mass), but they have no charge.
Those two facts make them very difficult to study. Since they have no charge, they don’t affect protons and electrons through electricity of magnetism. In fact, they go right through normal matter *better* than light goes through glass. They have almost no mass so gravity barely affects them and they always travel (almost) the speed of light. They have so little mass that for a long time we though they had no mass at all.
They do occasionally react in unusual ways meaning if you have a big enough thing (like an iceberg or the whole planet) you can detect them, but even so you will miss most of them.
What do they do? Mostly they just float around going in straight lines without caring what is there. They are important for a couple reasons. First, they are made in many nuclear reactions. If they aren’t made, the math doesn’t let the nuclear reactions happen. Second, they cause the outward explosion in supernovas. They have so little mass that they can escape collapsing stars. That is really important because something has to push some of the matter out. We, and everything else on our planet, are made of the stuff pushed out of exploding stars by neutrinos.
They don’t do much of anything, but they do balance the books in some kinds of fusion reactions and particle decay.
Fusion in stars destroys protons and electrons to make neutrons, there’s a physical law that says you get one electron neutrino in exchange for every electron destroyed. Beta decay creates electron-proton pairs from neutrons and kicks off anti-neutrinos.
If you have a really, really high neutrino or antineutrino flux it’s possible to observe them pushing those reactions backwards. The [Cowan-Reines](https://en.wikipedia.org/wiki/Cowan%E2%80%93Reines_neutrino_experiment) experiment used antineutrinos to make positrons. (Positrons don’t last very long but the annihilation reaction with electrons generates a distinctive gamma ray.)
This means they certainly play a meaningful role in supernovae and supernovae are needed to make any element heavier than iron, so they’re essential even though they can be ignored in pretty much every normal situation.
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