Back in the 1930s, physicists realized that some kinds of radioactive decay seemed to lose energy. Energy isn't supposed to disappear.
So they proposed that the energy might be going into some hitherto unknown kind of subatomic particle. This went on to be called the neutrino. It was a sort of filler: Its existence would mean that theoretically impossible energy loss wasn't actually happening, but no one had any direct evidence of its existence, or had any idea what its properties might be.
It wasn't actually discovered until a quarter of a century later. The 1956 experiment that confirmed neutrinos won the Nobel Prize in 1995.
But neutrinos still had more to say. They turned out to be the most abundant particle in the universe, other than light. Scientists assumed that (like light), they were totally massless.
But then they realized that neutrinos were disappearing.
Neutrinos are formed by lots of processes -- including some inside of our own bodies -- but the neutrinos created by our sun's nuclear reactions were proving to be problematic. Physicists were able to calculate how many particles should reach the Earth from the sun, and only about two thirds of them were showing up.
And the physicists were all like:
One possible explanation: More than one kind of neutrino. If we could only detect tortilla chips, we might not catch them as they turned into Doritos (regular and cool ranch, obviously).
The winners of the 2015 Nobel Prize led experiments that helped to confirm that these three flavor varieties exist. And by proving that neutrinos can oscillate between different types, they also proved that neutrinos must have some kind of mass.
Why do we care about that? Well, the Standard Model of physics assumed that neutrinos didn't have mass. This experiment provided the first evidence that the Standard Model might not be set in stone. Which isn't really surprising, given how science tends to work.
In the 80s, when people were just starting to get excited about the idea that neutrinos might have stowaway mass, some actually theorized that the mass of neutrinos might explain away dark matter. But even though neutrinos are incredibly abundant, their mass is too negligible for that to be the case.
Scientists are still working on calculating the exact mass of the particles, and on figuring out how their pervasiveness may have affected the birth and evolution of the universe.