Antimatter is in Mekanism v10 the final product of the Fissile Fuel cycle. it is the endgame material of V10. as such, it does not have a lot of uses as of now.
IT is mainly 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,