Antimatter

Antimatter is a powerful energy source, though somewhat unstable.

Fiction

Generation 1 cartoon continuity

Dr. Alcazar created an antimatter formula, which came to the attention of Megatron. Predictably, he wanted it as a source of energy. The doc deleted the formula, but sent it to Chip Chase. Ultimately, Megatron decided that antimatter was simply too unstable to use. Roll For It

Marvel Comics continuity

On the other hand, the Ark has an antimatter conversion chamber as part of its power system. All or Nothing!