antimatter

Definition: For every particle in nature, there is an anti particle - made of antimatter, with identical mass and spin, but with opposite electric charge, baryon number, strangeness, lepton number etc.

Antimatter was predicted by Dirac, based on strange "negative energy" solutions to his equation for an electron. Antielectrons were soon observed with exactly the properties Dirac predicted.

In 2002, antihydrogen was created by combining antiprotons and antielectrons.