Antimatter is in Mekanism v10 the final product of the Fissile Fuel cycle. it is the endgame material of V10. It is used in several crafting recipes such as Supermassive QIO drive and for units such as the Gravitational Modulating unit, the Teleportation Unit and the Elytra Unit.
It is also used by the antiprotonic Nucleosynthesizer to transmute certain blocks. it is crystallized in the Chemical Crystalizer with 1,000 MB of antimatter, and the resulting pellets are used to craft end game items.