Material composed of antiparticles, which correspond to ordinary protons, electrons, and neutrons but have the opposite electrical charge and magnetic moment. When matter and antimatter collide, both may be annihilated, and other elementary particles, such as photons and pions, are produced.

In 1932 Carl D Anderson, while studying cosmic rays, discovered the positron or antielectron, the first known as antiparticle.

Any antimatter in our part of the universe is necessarily very short lived because of the overwhelming preponderence of ordinary matter, by which the antimatter is quickly annihilated.