Antimatter

Antimatter is a highly dangerous substance that releases a massive amount of energy when coming into contact with matter, such as air, thus causing an explosion. Antimatter is known to occur naturally in extremely small amounts, but can be made artificially in larger, but still extremely small amounts by Humans, however the Covenant are suspected as having a large amount of either natural or artificial Antimatter to use as weaponry and possibly a power-source. Humanity is thought to currently have Antimatter, but apparently not enough for it to be used as weaponry. The Covenant, however have used Antimatter as weaponry on more than one occasion, but how they get their Antimatter remains a mystery.

Uses

 * Antimatter Charge
 * Covenant Bomb

Trivia

 * Anti-Matter is a real substance.
 * Several other Science Fiction sagas and stories, besides Halo, use Anti-Matter, such as Angels & Demons.