In particle physics, antimatter extends the concept of the antiparticle to matter, whereby antimatter is composed of antiparticles in the same way that normal matter is composed of particles. For example an antielectron (positron) and an antiproton could form an antihydrogen atom in the same way that an electron and a proton form a normal matter hydrogen atom. Furthermore, mixing of matter and antimatter would lead to the annihilation of both in the same way that mixing of antiparticles and particles does, thus giving rise to high-energy photons (gamma rays) or other particle–antiparticle pairs. The particles resulting from matter-antimatter annihilation are endowed with energy equal to the difference between the rest mass of the products of the annihilation and the rest mass of the original matter-antimatter pair, which is often quite large. Antimatter is not found naturally on Earth, except very briefly and in ephemerally small quantities (as the result of radioactive decay or cosmic rays). This is because antimatter which comes to exist on Earth outside the confines of a suitably equipped physics laboratory would inevitably come into contact with the ordinary matter that Earth is made of, and be annihilated. Antiparticles and some stable antimatter (such as antihydrogen) can be made in minuscule amounts, but not in enough quantity to do more than test a few of its theoretical properties. There is considerable speculation both in science and science fiction as to why the observable universe is apparently almost entirely matter, whether other places are almost entirely antimatter instead, and what might be possible if antimatter could be harnessed, but at this time the apparent asymmetry of matter and antimatter in the visible universe is one of the great unsolved problems in physics. Possible processes by which it came about are explored in more detail under baryogenesis. Source: http://en.wikipedia.org/wiki/Antimatter

When quantum mechanics was discovered & developed in the early 20th century, it turned out that the equations predicted, for each particle, a so-called anti-particle counterpart. Antiparticles have all the same properties (spin, mass, etc) except that electric charge is opposite. Thus the electron (charge = -1) has an antiparticle known as a positron (charge = +1), predicted by Dirac prior to its discovery. Protons (charge = +1) have antiparticles known as anti-protons (charge = -1), and so on. Neutral particles such as neutrons or photons are their own antiparticles. This raises the intriguing possibility of anti-matter atoms formed by nuclei (consisting of neutrons and anti-protons) bound to positrons (which play the role of electrons), which pretty much follow the rules for atoms -- allowing anti-molecules and even anti-matter galaxies, stars, & planets. No such objects have been observed, however. That's good, because when a particle and its anti-particle combine, they annihilate each other in a burst of pure radiative energy. One of the central cosmological mysteries is why the universe seems to be predominantly matter, rather than equal parts of matter and anti-matter.

EASY VERSION: Ok. I am no expert i am only 13. But i know antimatter seemed to have collided with matter which caused the Big Bang. Only tiny amounts of antimatter is emitted from the centre for our galaxy, the milky way.Antimatter and matter collide and form massive amount of energy. In space, they r released in the form of gamma rays. (ALOT of energy) 13MGv i think or 130Mgv Did u know we u antimatter is used in CAT Scans? It is because when injected into the brain, the swollen parts of it have more matter than usual right? Then the antimatter will react with the EXTRA matter, therefore giving off a LOT of energy from the SWOLLEN areas. CAT scan machine will use the energy to form a somewhat map of ur brain. Thanks to antimatter we can be treated quickly. Antimatter is extremely limited. Perhaps when the big bang occured there was more matter than antimatter. Scientists r not really sure yet. Hope this has been helpful! :D

Simple answer: Matter with the charges switched around