An equation implied a mirror electron — then it turned up in a cloud chamber

In 1928, Paul Dirac's equation unifying quantum mechanics and relativity produced a strange extra solution: a particle identical to the electron but positively charged. Most physicists assumed it was a flaw in the math. In 1932, Carl Anderson photographed exactly such a particle in cosmic-ray tracks, confirming antimatter was real.

— Carl D. Anderson, The Positive Electron — Physical Review, 1933