As its name implies, antimatter is the opposite of regular matter. The positron, for example, is a positively charged equivalent of the negatively charged electron. When antimatter comes in contact with regular matter, it explodes.
Antimatter's existence was first proposed in 1928 by physicist Paul Dirac. He believed that for every particle that exists there is a corresponding antiparticle, exactly matching the particle but with an opposite charge. This is where science fiction gets the notion of an antimatter universe that is the mirror image of our own.
According to Dirac, for every electron with a negative charge, there is a corresponding antielectron with a positive charge - a positron. For every proton with a positive charge, there is an antiproton with a negative charge.
The first science fiction story to deal with antimatter was probably "Minus Planet" by John D. Clark, published in the April 1937 issue of Astounding Stories. Antimatter also played a key role in the "seetee" stories published in the 1940s by Jack Williamson.
In 1943, A.E. Van Vogt wrote "Storm," in which a huge storm in space is caused by a gas cloud of ordinary matter coming into contact with an antimatter gas cloud. And in 1950, J. Bridger wrote a story "I Am a Stranger Here Myself" in which mankind learns from aliens how to travel faster than the speed of light using a "multi-phase travel" technology based on transforming matter into antimatter - a precursor of the U.S.S. Enterprise's warp engines.