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• Neutrinos are one of the most elusive and omnipresent particles in the universe. Their strange properties may explain why the amounts of matter and antimatter in our universe aren’t equal, as most models of the universe say they should be.
• The problem is that if they’re equal, matter and antimatter in the young Big Bang would have annihilated each other and the universe would be empty.
• A large-scale experiment showed that neutrinos can violate a fundamental law of nature, tipping the scales in favor of matter prevailing.

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In the forests of far northern Minnesota a team of scientists are hunting for ghosts. But not the paranormal shades of dead relatives. They’re looking for one of the most elusive particles in the universe. A particle so critical that it may hold the keys to explaining why anything exists at all.

We call them neutrinos. They were first discovered in the early 20th century when physicists noticed that some nuclear reactions didn’t quite add up: the energy and momentum of the inputs didn’t equal the outputs. The answer was a new particle, previously unknown, that had slipped by undetected as a product of the reactions. Legendary physicist Enrico Fermi gave neutrinos their name, an Italian-ish word for “little neutral ones.”

Trillions of neutrinos pass through your body every second. They are manufactured in every nuclear reaction in the entire universe. They are made in distant supernova explosions. They are made in the core of the Sun. They are even made in our nuclear power plants. But they so rarely interact with matter that despite the flood that passes through you at all moments, you are unlikely to interact with even a single one in your whole life.

The scientists in northern Minnesota were part of the NOvA collaboration, and they had built a neutrino detector the size of a double-decker Airbus A380. They watched as neutrinos generated in Fermilab, just outside Chicago and over 800 kilometers away, raced through the Earth and slipped through their detector. After years of observations and trillions upon trillions of neutrinos, the team managed to detect a few hundred of them.

It was enough to illuminate their peculiar quality.

Neutrinos have a special property that might just explain a strange characteristic of our universe: the fact that anything exists at all. The problem is that in almost all of our calculations and theories and models of how the universe works, matter and antimatter should come in exactly equal proportions.

Antimatter is just like regular matter but with opposite charge. A positron, the antimatter of the electron, has the exact same mass and spin, but is (as its name suggests) positively charged. When matter and antimatter meet, they annihilate each other in a flash of pure energy.

Similarly, any particle interaction or reaction that produces regular matter has a twin version that produces antimatter, and the two kinds of reactions are just as likely to occur. So there should be an equal balance between antimatter and matter in the universe.

But there isn’t. Where did all the antimatter go?

The question remains one of the greatest unsolved puzzles in modern cosmology. Something in the early universe—some strange, exotic process—must have tipped the balance in favor of matter. It didn’t have to be much. Matter just had to win out over antimatter at a ratio of one to a billion. But it had to have been quick, otherwise all the matter and antimatter in the young Big Bang would have annihilated each other in furious blasts of energy, leaving nothing but an empty universe behind.

Many scientists believe that neutrinos might hold the answer to this perplexing riddle, and that’s because they have one other trick up their sleeves: the ability to change their identity.

There isn’t just one kind of neutrino. There are three, called the electron neutrino, the muon neutrino, and the tau neutrino. Unlike other particles, as neutrinos travel they can cycle amongst these different identities, phasing in and out of each kind. So if a nuclear reaction produces a lot of one kind of neutrino, a detector set hundreds of miles away will find a lot fewer than predicted of that kind.

By comparing how many neutrinos we know the reactions produce, to how many are observed, physicists can get a handle on what other games the neutrinos might be up to.

In a paper published in Nature in October, the NovA experiment in collaboration with a similar experiment in Japan laid out the results of their years-long study of neutrinos. They found the possibility that neutrinos can violate one of the most fundamental symmetries in all of nature: the symmetry of charge.

Usually in particle interactions, the amount of charge going into the interaction must equal the amount going out. This is why matter and antimatter must remain in balance. But neutrinos might be able to violate this, allowing interactions where the total amount of charge is different from beginning to end.

This opens a doorway to the early universe, where an absolute flood of neutrinos may have triggered a cascade of reactions that led to matter prevailing over antimatter.

As scientists expected, the new results are far from conclusive, given the ghostly and slippery nature of neutrinos. But they do point in an enticing direction.

Who knew the explanation for why there is something rather than nothing may lie in a particle that we barely even knew existed in the first place?

Paul M. Sutter

Contributor, science educator and theoretical cosmologist

Paul M. Sutter is a science educator and a cosmologist at Johns Hopkins University and the author of How to Die in Space: A Journey Through Dangerous Astrophysical Phenomena and Your Place in the Universe: Understanding Our Big, Messy Existence. Sutter is also the host of various science programs, and he’s on social media. Check out his Ask a Spaceman podcast and his YouTube page.