Antimatter is matter that is composed of the antiparticles of those that constitute normal matter. An atom of anti-hydrogen, for instance, is composed of a negatively charged antiproton being circled by a postitively charged positron. If a particle/antiparticle pair comes in contact with each other, the two annihilate in a burst of energy.
With antimatter, the entire possible energy of the matter could be harnessed, instead of the very small chemical energies or nuclear energies that can be extracted today. Since the energy density is vastly higher than these other forms, the thrust to weight equation used in antimatter rocketry and spacecraft would be very different. In fact, the energy in a few grams of antimatter is enough to transport a small ship to the moon. A matter, antimatter annihilation reaction turns the entire mass of the matter and antimatter into energy. This is far more efficient than even a nuclear fusion reaction. It is hoped that antimatter could be used as fuel for interplanetary travel or possibly interstellar travel, but it is also feared that if humanity ever gets the capabilites to do so, there could be the construction of antimatter weapons.
As far as we know there are no antimatter atoms in existence in this universe outside of our particle physics labs. This is a great mystery since one would expect matter and antimatter to have been generated in equal amounts after the Big Bang. The scarcity of antimatter has given us a stable universe, however, without which life could not have evolved.
The scarcity of antimatter means that it is not readily available to be used as fuel. Generating a single atom of antimatter is immensely difficult and requires particle accelerators and vast amounts of energy - millions of times more than is released after it is annihilated with ordinary matter, due to inefficiencies in the process. No more than a handful of antimatter atoms have ever been made. Therefore, unless substantial quantities from some as-yet unimagined natural source of antimatter are found, or ways to generate antimatter more efficiently are determined, antimatter will remain a curiosity rather than a viable propulsion system.
The symbol used to denote an antiparticle is the same symbol used to denote its normal matter counterpart, but with an overstrike. For example, a proton is denoted with a "p", and an antiproton is denoted by a "p" with a line over its top ().
See also: ambiplasma