If equal amounts of matter and antimatter in fact existed, they would promptly combine and annihilate each other in a burst of energy. Indeed, a particle and its antiparticle are equal yet opposite in every way and should behave similarly. Physicists call this property symmetry. The fundamental symmetries of nature include charge and parity, or handedness. So an electron should behave the same way as a positron (its antiparticle), just as a particle in a right-handed coordinate system should do the same things if viewed in a three-dimensional mirror, thereby putting it in a left-handed system. Because our matter-dominated universe exists, though, physicists have been looking for situations in which particles and their antiparticles break from symmetry.
http://www.scientificamerican.com/explorations/2001/082001antimatter/
