Neutrinos are tiny sub-atomic electrically neutral particles. They are produced by extreme nuclear reaction like those that happen in the core of the sun or an exploding star. There are trillions of them flying through us every second but they are really hard to find and they hardly ever hit our atoms. In fact it is so small that it has a mass million times less than that of an electron which is plentifully minuscule and also as it is electrically neutral it does not react with charged particle in any sort of way.
Even an extremely huge neutrino detector will only detect a few neutrinos a day. Even so, scientist mostly understand why they don’t interact with ordinary matter but what they don’t understand is why neutrinos have mass or why it is so small. Particle physicists use the standard model which uses math to describe how every known particle interacts with every other known particle. It is one of the most successful model in the history. It correctly predicts the results of literally trillions of experiments. But the problem is the standard model also predicts that neutrinos shouldn’t have any mass.
But in the 1990’s, physicists studying neutrinos coming from the sun realized that neutrinos had to have mass. There are a few different kinds of neutrinos, and the researchers found out that the neutrinos coming from the sun were switching types. But they would need mass to do that switching which means that the standard model has a pretty large hole in it.
Though there is a way of changing the equation used in the standard model so that it includes neutrinos with masses. So, when its on its own, the fact that neutrinos have mass doesn’t necessarily have to be a deal breaker. But, neutrinos masses are incredibly tiny compared to every other fundamental particle out there.
As mentioned earlier, neutrinos have mass million times less than that of electrons and electrons are the next lightest particle we have found and they are still somewhere between 126,000 and 600 million times heavier than the lightest neutrinos. That huge gap makes physicist think that fitting neutrinos with mass into the current standard model is quite unreasonable. There a few other explanations that also fit with the standard model but so far we haven’t found any solid evidence to support them. Other physicists think that we need to throw the standard model altogether and turn to new model to explain the mysterious masses.