Antimatter consists of antiparticles, which possess the same mass as their ordinary matter counterparts but with opposite physical charges, such as the positively charged positron (antielectron). When a particle meets its antiparticle, they undergo annihilation, converting their mass into energy, typically photons, a process exploited in medical imaging like positron emission tomography. Antiparticles can be created naturally, for instance, in radioactive decay or cosmic ray collisions, or in particle accelerators through "pair production" where both a particle and its antiparticle are simultaneously generated.
The existence of the positron was predicted by Paul Dirac and experimentally confirmed by Carl D. Anderson in 1932, while the antiproton and antineutron were discovered by Emilio Segrè and Owen Chamberlain in 1955. A major unanswered question in physics is why the universe, despite the near-perfect symmetry between matter and antimatter, consists almost entirely of matter following the Big Bang, with charge parity violation offering some, though incomplete, explanations.