Antimatter
Like Superman's alter-ego, Bizzaro, the particles making up normal matter also have opposite versions of themselves. An electron has a negative charge, for example, but its anti-matter equivalent, the positron, is positive. Matter and anti-matter annihilate each other when they collide and their mass is converted into pure energy by Einstein's equation E=mc2. Some futuristic spacecraft designs incorporate anti-matter engines.
Credit: Penn State U. /NASA-MSFC