Antimatter is made of particles that share the mass of ordinary matter but carry opposite electric charge and other quantum numbers. When a particle meets its antiparticle the two annihilate, converting their entire mass into energy. The Big Bang should have produced matter and antimatter in equal measure, and yet the universe we observe is built almost entirely from matter. Accounting for that imbalance, a problem known as baryogenesis, remains one of the central unsolved questions in physics.