From Scratchpad

Corresponding to most kinds of particle, there is an associated antiparticle with the same mass and opposite electric charge. (The exceptions are massless gauge bosons such as the photon.) Even electrically neutral particles, such as the neutron, are not identical to their antiparticle. In the example of the neutron, the 'ordinary' particle is made out of quarks and the antiparticle out of antiquarks. Antiparticles compose antimatter, which annihilates itself and its matter counterpart upon coming upon this matter counterpart. Annihilation converts both the matter and antimatter into massive amounts of energy, leading to the use of antimatter as a powerful weapon.

[edit] Antimatter Warfare

Antimatter is employed as a weapon by several advanced civilizations, most notably the Xam Empire. Starships utilizing antimatter as weaponry are often called "annihilators."