Antimatter is a form of mass that annihilates normal matter as well as itself if the two are ever brought into contact. Antimatter can be found in minute quantities in nature, such as the upper atmosphere of planets and certain kinds of radioactive decay, but it is destroyed by the surrounding matter almost instantly and therefor not usable for harvesting. With an annie-plant and the right technology, it can be created artificially. Due to the release of energy created by matter-antimatter annihilation, antimatter can be used as a fuel or an explosive, and in a few specialized applications this can even be done economically.
Antimatter is stored by encapsulating single antiprotons or antihelium ions inside fullerene molecules. The resulting "fullerened antimatter" is a black powder which contains antimatter at the parts-per-thousand level and can be safely stored with no additional containment, although it should NOT be incinerated. A stockpile of fullerened antimatter nearly destroyed Credomar Habitat when the building it was stored in caught fire. It's possible to make fullerened antimatter with larger anti-atoms if they can be acquired, allowing ambitious users to pack truly absurd yields into small munitions.
Antimatter is used to power breacher munitions and similar one-shot devices too small to include an annie-plant. Its use is frowned on in munitions that could use annie-plants instead, since the yield of an AM explosive cannot be controlled. Despite the name, neutronium-annihilation powerplants use gravy for fuel conversion, not antineutronium.