antimatter

n. matter composed of counterparts of ordinary matter (Physics)

For other senses of this term, see antimatter (disambiguation). In particle physics and quantum chemistry, antimatter extends the concept of the antiparticle to matter, whereby antimatter is composed of antiparticles in the same way that normal matter is composed of particles. For example an antielectron (a positron, an electron with a positive charge) and an antiproton (a proton with a negative charge) could form an antihydrogen atom in the same way that an electron and a proton form a normal matter hydrogen atom. Furthermore, mixing of matter and antimatter would lead to the annihilation of both in the same way that mixing of antiparticles and particles does, thus giving rise to high-energy photons (gamma rays) or other particle–antiparticle pairs. The particles resulting from matter-antimatter annihilation are endowed with energy equal to the difference between the rest mass of the products of the annihilation and the rest mass of the original matter-antimatter pair, which is often quite large.
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A particle with precisely opposite properties to those of its matter counterpart. For example, the antimatter counterpart of an electron, known as a positron, has a positive charge, equal in strength to the negative charge of the electron. It also has an opposite spin. Antimatter is created in a process known as pair production. When antimatter comes into contact with its matter counterpart, the two particles are instantly annihilated by having their rest mass turned into energy and released as two photons electromagnetic radiation. The universe is predominantly composed of matter, since the laws of physics state that matter is produced with a slight but calculable bias over antimatter.

Noun
1. matter consisting of elementary particles that are the antiparticles of those making up normal matter
(hypernym) substance, matter
(substance-meronym) antiparticle