Antimatter

Antimatter is matter which is composed of the antiparticles of "ordinary" matter. A collision between any particle and its anti-particle partner leads to their mutual annihilation, which can release vast amounts of energy.

It is extremely expensive and difficult to produce and store and may be collected from naturally occurring radiation belts around planets.

Isolated and stored anti-matter could be used as a fuel for interplanetary or interstellar travel as part of an antimatter catalyzed nuclear pulse propulsion or other antimatter rocketry.

It is also the power source for antimatter weapons.