antimatter
From Trekipedia
Antimatter is composed of subatomic particles of the same mass as normal matter, but with an opposite charge; an antiproton, for example, has the same mass as a proton, but with a negative charge. When antimatter particles come into contact with corresponding particles of matter, the result is mutual annihilation. All of the mass of the particles is converted into energy in the form of gamma radiation, the most energetic photons in the electromagnetic radiation spectrum. When electrons and positrons collide, the result is a burst of gamma rays whose energy is 511 million electron volts (MeV).[1]
