Matter composed of antiparticles: subatomic particles that have identical rest mass to corresponding particles of ordinary matter but opposite charge, and are opposites in other fundamental properties. For example, the antiparticle of the electron is the positron, which has a positive charge equal to the electron's negative charge; the antiproton has a negative charge equal to the proton's positive charge. When matter and antimatter meet, they annihilate each other, releasing energy. The Universe seems to be almost entirely in the form of matter rather than antimatter; why this should be so is presumably related to events shortly after the Big Bang.
Subjects: Astronomy and Astrophysics.