The universe shouldn’t be here. Everything scientists know about particle physics, summed up in a theory called the Standard Model, suggests that the big bang should have created equal quantities of matter and antimatter. A mirror version of matter, antimatter consists of partner particles for all the regular particles we know of, equal in every way but with opposite charge. When matter and antimatter particles collide, they destroy one another, so the mass created when the universe was born should have been completely wiped out, leaving an empty, featureless cosmos containing only light. That there was enough leftover matter after this great annihilation to form galaxies, stars, planets and even us but almost no antimatter is known as the matter-antimatter imbalance. This existential anomaly is one of the great outstanding mysteries of modern physics.

Physicists have concocted many hypotheses to explain this mismatch, but we don’t know which, if any, are true. Some of them seek to offer matter the upper hand by introducing new particles that decay, producing more matter than antimatter in the process, or that interact differently with matter and antimatter. And some of these proposals include side effects that scientists can hope to detect, thereby providing evidence for the theories. One example is an exotic property of electrons called the electric dipole moment, a small difference between the center of mass of an electron and its center of charge. Such a displacement has never been detected and should be much smaller than current experiments could measure. But many proposed extensions to the Standard Model that seek to explain the matter-antimatter imbalance predict much larger values for the electric dipole moment.