Although antimatter is always being mentioned in science-fiction books, and being given all sorts of strange powers, it is an established scientific fact, and not nearly as odd as you might think. However, the presence or absence of it in the cosmos around us has important repercussions for our ideas of how the Universe was created.
Antimatter is really just like normal matter, only a mirror-image of it.
In theory, every type of particle that makes up the matter around us has its own antiparticle, and these bond together to make large-scale antimatter, in just the same way as normal fundamental particles bond together to make atoms and larger matter. One important thing we do know about antimatter is that whenever a particle and its antiparticle come into contact with each other, they annihilate - destroying each other completely, and producing a burst of energy. This is the basis for antimatter drives on spaceships such as the USS Enterprise in Star Trek. From an astronomical point of view, the interesting thing about antimatter is how little of it there is around. It had been suggested that our part of the Universe, which inflated during the Big Bang, expanding at an incredible rate to dominate the rest, might have contained more matter than antimatter.