antimatter

Antimatter is a rare, separate type of matter formed of particles with some of their quantum numbers being the opposite of those in corresponding normal matter, among these differing quantum numbers being electrical charge. When a normal-matter particle encounters its antimatter analog particle (antiparticle), they destroy each other (known as particle-antiparticle annihilation, or just annihilation), the energy being converted to other particles, generally photons.

Antimatter is extremely rare throughout the solar system and there is evidence that this is true of the entire observed universe, i.e., the universe has a matter-antimatter asymmetry. The annihilation reaction can be detected by the extremely high energy photons that annihilation produces. Cosmic rays, which apparently come from very distant parts of the universe, do not produce enough such photons when striking the Earth's atmosphere to suggest anywhere near an equal balance among the sources of the particles, nor are such photons from distant sources sensed in numbers that would indicate many incidents of annihilation in such distant locations. There is no consensus as to this asymmetry's cause, though cosmological theories term the causation event baryogenesis, presumed to have occurred in the very early universe.

Antimatter particles include the positron (i.e., antielectron, like an electron but positively charged), the antiproton, the antineutron, antineutrinos, and antiquarks. The particles can be combined to form "anti atoms", e.g., antihydrogen, and this has been done experimentally.

Positrons and antineutrinos are commonly produced by beta decay, thus in nucleosynthesis, and other antimatter particles are commonly formed by high-energy collisions, e.g., in particle accelerators, and by cosmic rays striking ordinary matter. The particles (other than antineutrinos) are generally quickly annihilated when surrounded by normal matter.