- Matter of the opposite charge (or more rarely magnetic spin or other characteristic) to the equivalent particles making up the bulk of the mass of the present universe. Examples of antimatter include positrons – identical to electrons except for having a positive rather than negative electrical charge – and antiprotons, which are identical to protons except for having a negative rather than positive electrical charge. Little is known about why there is so little antimatter in the universe we see around us, but when matter and antimatter meet they annihilate each other with a release of energy.
- Matter composed of antiparticles such as antiprotons, antineutrons, and positrons. When antimatter meets its conventional matter counterpart, they are each annihilated, releasing a large quantity of energy. Also spelled antimatter.