Antimatter is the negative opposite of normal matter, of which nearly everything in the universe is composed. Just as normal matter is made up of electrons orbiting a nucleus composed of protons and neutrons, so antimatter consists of anti-electrons (or positrons) orbiting around a nucleus of anti-protons and anti-neutrons.
The existence of antimatter was first theorized in 1928 by the physicist Paul Dirac while he was trying to integrate Albert Einstein's Theory of Special Relativity with the Quantum Theory of Erwin Schrodinger and Werner Heisenberg. By 1932, Paul Anderson had discovered the first of the three elementary anti-particles, the anti-electron, which he named the positron. In 1955, the anti-proton was discovered, followed just a few years later by the anti-neutron in 1959. The first anti-deuterion, which is composed of a single anti-proton and single anti-neutron, and is the nucleus of anti-deuterium, was created in 1965. A team working at CERN in 1995 was able to combine anti-deuterions with positrons to form the first anti-deuterium atoms.
Antimatter is important because when an atom of normal matter is brought in contact with an atom of antimatter, the two annihilate each other and generate a large release of energy. This energy is harnessed, for example, by a starship's warp engines to generate its warp field.
Because antimatter will react with any normal matter it comes in contact with, the precautions with which it is handled are extreme. Antimatter is always contained within a strong magnetic field, preventing accidental release.The only normal matter allowed to contact antimatter is the fuel (typically deuterium in a modern warp reactor), and only dilithium crystals, which moderate the matter-antimatter reaction, are exposed to an antimatter stream.