The antimatter counterpart of the proton, encountered as a destructive beam weapon and as a scanning method that the Dominion and the Cardassians used to hunt cloaked starships.
An antiproton is a proton's antimatter twin, and traces of antiprotons are a familiar sign of weapons fire or of a warp vessel's passing. The planet killer of the Original Series episode The Doomsday Machine cut worlds apart with a beam of pure antiprotons.
In Deep Space Nine the particle becomes a way of defeating cloaks. In The Search, Part I, the Jem'Hadar sweep space with an antiproton beam and eventually find the cloaked USS Defiant, and in Defiant the Cardassians use the same technique while pursuing Thomas Riker after he steals the ship.
The method showed that the borrowed Romulan cloaking device aboard the Defiant was far from a perfect defence in the Gamma Quadrant.