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, theoretically releasing one hundred percent (100%) of all matter held within the annihilating particles; mainly in the form of Gamma rays (photons) and other matter/antimatter particles. To put this into perspective, the energy released in a hydrogen bomb is only 0.007% of the energy within those particles.
The Covenant use antimatter as a reactant in ordnance, namely antimatter charges. Certain Forerunner weapons use antimatter as well; the Z-390 incineration cannon, for example, fires antimatter embedded within streams of energized particles.