antimatter

Antimatter is made of antiparticles — counterparts of ordinary particles that share the same mass but carry opposite charges and certain other reversed properties. The antiparticle of the electron is the positron, with the same mass but positive rather than negative charge; the proton has the antiproton, and so on. Antimatter is not science fiction: it is produced routinely in laboratories and even trickles out of some natural radioactive decays.

Its existence was a prediction before it was a discovery, which is part of what makes it remarkable. When Dirac wrote down a wave equation that respected both quantum mechanics and special relativity, the mathematics insisted on extra solutions with negative energy. Rather than discard them, he reinterpreted them as particles of opposite charge, and a few years later the positron was found in cosmic rays exactly as foretold.

When a particle meets its antiparticle they can annihilate, converting their entire mass into energy, usually as a burst of high-energy photons. This is the most efficient energy release known, which is why antimatter recurs in speculation about future propulsion, though making and storing more than minuscule amounts is utterly impractical today. A deep open question is why the universe is made overwhelmingly of matter, with almost no antimatter left over from the Big Bang.