The Delta Rana warship firing a beam of jacketed antiprotons and positrons
An antiproton was the antimatter counterpart of a proton.
A beam of pure antiprotons was an incredibly destructive weapon. Such a weapon was used by the so-called "planet killer" to destroy planets. (TOS: "The Doomsday Machine")
The Delta Rana warship, created by the Douwd Kevin Uxbridge, used jacketed beams of antiprotons and positrons as its main weapon. It fired beams with a power of forty megawatts, and later four hundred gigawatts, at the USS Enterprise-D. (TNG: "The Survivors")
The Crystalline Entity left behind a trail of antiprotons, by which it can be tracked across space. (TNG: "Silicon Avatar")
Romulan disruptors left a characteristic trace of residual antiprotons, which could be used to date when the weapon was fired. (TNG: "Face of the Enemy")
The deflector dish of Sovereign-class starships was charged with antiprotons. (Star Trek: First Contact)
The Jem'Hadar used antiproton beams to detect cloaked ships. (DS9: "The Search, Part I")
In 2372, The Doctor on the USS Voyager used bursts of antiprotons from the warp core to reverse the effects of hyper-evolution on Tom Paris and Kathryn Janeway, by destroying the mutated segments of DNA. (VOY: "Threshold")
The use of a cloaking device often left residual antiprotons that could expose the ship's presence. The Reman warbird Scimitar was equipped with an advanced cloaking device that left no such trace. (Star Trek Nemesis)