What is Antimatter?

Antimatter particles are sub-atomic particles with properties opposite those of normal matter particles. For example, a positron is the antiparticle equivalent of the electron and has a positive charge. When a particle and its antiparticle meet, they annihilate, releasing massive amounts of energy, according to Einstein's famous equation E=mc^2, where E is equal to energy, m is equal to mass, and c is the speed of light.
Antimatter particles are created in ultra high-speed collisions. In the first moments after the Big Bang, only energy existed. As the universe cooled and expanded, particles of both matter and antimatter were produced in equal amounts.
But antimatter is rare in today's universe. Scientists aren't sure why. One theory suggests more normal matter was created than antimatter in the beginning, so that even after mutual annihilation there was enough normal matter left to form stars, galaxies and us.
The phenomenon was first predicted in 1928 by English physicist Paul Dirac. He first proposed the existence of antiparticles when he derived equations which would work for an electron with a negative charge and an electron with a positive charge—an antiparticle. His predictions were confirmed with experiments in 1932 by American physicist Carl Anderson.