Everything we see around us, from the ground beneath our feet to the most remote galaxies, is made of matter. For scientists, that has long posed a problem: According to physicists’ best current theories, matter and its counterpart, antimatter, ought to have been created in equal amounts at the time of the Big Bang. But antimatter is vanishingly rare in the universe. So what happened?
Physicists don’t know the answer to that question yet, but many think the solution must involve some subtle difference in the way that matter and antimatter behave. And right now, the most promising path into that unexplored territory centers on new experiments involving the mysterious subatomic particle known as the neutrino.
“It’s not to say that neutrinos are definitely the explanation of the matter-antimatter asymmetry, but a very large class of models that can explain this asymmetry are connected to neutrinos,” says Jessica Turner, a theoretical physicist at Durham University in the United Kingdom.
Let’s back up for a moment: When physicists talk about matter, that’s just the ordinary stuff that the universe is made of—mainly protons and neutrons (which make up the nuclei of atoms), along with lighter particles like electrons. Although the term “antimatter” has a sci-fi ring to it, antimatter is not all that different from ordinary matter. Typically, the only difference is electric charge: For example, the positron—the first antimatter particle to be discovered—matches an electron in its mass but carries a positive rather than a negative charge. (Things are a bit more complicated with electrically neutral particles. For example, a photon is considered to be its own antiparticle, but an antineutron is distinct from a neutron in that it’s made up of antiquarks rather than ordinary quarks.)