From Halopedia, the Halo wiki
Antimatter is the physical counterpart to matter. Antimatter consists of antiparticles which have the same mass as their matter counterparts but with a reversed charge. Antimatter annihilates upon contact with matter.
The Covenant use antimatter as a reactant in ordnance, namely antimatter charges. These charges were used in the Battle over Installation 04 and to destroy Athens Station, Malta Station, and nearly Cairo Station in the Battle for Earth.
Certain Forerunner weapons use antimatter as well; the Z-390 incineration cannon, for example, fires antimatter embedded within streams of energized particles. Converging beam cannons of the Sojourner-class dreadnought created paths for antimatter to pursue enemy starships.