In physics, antimatter is matter composed of antiparticles that have the same mass as particles of ordinary matter, but have some quantum numbers, such as electric charge, of opposite sign. The laws governing the combination of antiparticles to form antielements (or antiatoms) and antimolecules are symmetrical to those governing matter.
When a particle and an antiparticle come into contact, the phenomenon of annihilation occurs, i.e. the matter involved is transformed into electromagnetic radiation in the form of high-energy photons (gamma rays), or into other particle-antiparticle pairs, so that the sum of the total energy before and after the event remains constant, according to the principle of conservation of mass and energy. Under certain conditions, particles and antiparticles can, for very short periods of time, become unstable particles, such as mesons, or an exotic atom, such as positronium.
Although it is believed that matter and antimatter were originally equivalent, there is detectable antimatter in small amounts in the present universe, some of which is produced by experiments and very quickly annihilated by matter. The reason for the predominance of matter is the subject of active research.