n.
Substance composed of elementary particles having the mass and electric charge of ordinary matter (such as electron s and proton s) but for which the charge and related magnetic properties are opposite in sign.
The existence of antimatter was posited by the electron theory of P.A.M. Dirac . In 1932 the positron (antielectron) was detected in cosmic ray s, followed by the antiproton and the antineutron detected through the use of particle accelerator s. Positrons, antiprotons, and antineutrons, collectively called antiparticles, are the antiparticles of electrons, protons, and neutrons, respectively. When matter and antimatter are in close proximity, annihilation occurs within a fraction of a second, releasing large amounts of energy .