Antimatter is the volatile counterpart to ordinary matter whose annihilation with matter supplies the enormous energy needed to power warp-capable starships.
When antimatter meets ordinary matter, both are converted almost entirely into energy. Starships store antimatter in magnetic containment pods and feed it into the warp core under tight control, since any failure of containment can destroy a vessel outright. Antimatter is also the explosive charge in photon torpedoes.
Captain Kirk once used a measured quantity of antimatter as bait and as a weapon against a deadly gaseous creature that fed on blood, luring it to Tycho IV and destroying it in the resulting explosion.