A very common type of power plant especially in the eastern parts of the Galaxy, an anti-matter reactor generates energy by colliding particles of matter and anti-matter which annihilate each other. The process is a very efficient source of power, but anti-matter fuel is rare and pricy, especially compared to cheap hydrogen fusion. Nevertheless, most civilizations are willing to pay the price of gathering anti-matter, since the power output for standard fusion plants are often not enough to supply power to all spacecraft systems, in particular aboard military starships engaged in combat action.
An antimatter reactor is the most efficient "conventional" means of producing energy, but their use is usually limited to starships, orbital installations and similar isolated settings. Planetary use is heavily discouraged by most governments as the risks of a large-scale anti-matter containment failure on a planetary surface are catastrophic, potentially resulting in the destruction of the entire planetary body.