Antimatter came about as a solution to the fact that the equation describing a free particle in motion (the relativistic relation between energy, momentum and mass) has not only positive energy solutions, but negative ones as well! If this were true, nothing would stop a particle from falling down to infinite negative energy states, emitting an infinite amount of energy in the process--something which does not happen. In 1928, Paul Dirac postulated the existence of positively charged electrons. The result was an equation describing both matter and antimatter in terms of quantum fields. This work was a truly historic triumph, because it was experimentally confirmed and it inaugurated a new way of thinking about particles and fields.
In 1932, Carl Anderson discovered the positron while measuring cosmic rays in a Wilson chamber experiment. In 1955 at the Berkeley Bevatron, Emilio Segre, Owen Chamberlain, Clyde Wiegand and Thomas Ypsilantis discovered the antiproton. And in 1995 at CERN, scientists synthesized anti-hydrogen atoms for the first time.
When a particle and its anti-particle collide, they annihilate into energy, which is carried by "force messenger" particles that can subsequently decay into other particles. For example, when a proton and anti-proton annihilate at high energies, a top-anti-top quark pair can be created!
An intriguing puzzle arises when we consider that the laws of physics treat matter and antimatter almost symmetrically. Why then don't we have encounters with anti-people made of anti-atoms? Why is it that the stars, dust and everything else we observe is made of matter? If the cosmos began with equal amounts of matter and antimatter, where is the antimatter?
Experimentally, the absence of annihilation radiation from the Virgo cluster shows that little antimatter can be found within ~20 Megaparsecs (Mpc), the typical size of galactic clusters. Even so, a rich program of searches for antimatter in cosmic radiation exists. Among others, results form the High-Energy Antimatter Telescope, a balloon cosmic ray experiment, as well as those from 100 hours worth of data from the Alpha Magnetic Spectrometer aboard NASA's Space Shuttle, support the matter dominance in our Universe. Results from NASA's orbiting Compton Gamma Ray Observatory , however, are uncovering what might be clouds and fountains of antimatter in the Galactic Center.
We stated that there is an approximate symmetry between matter and antimatter. The small asymmetry is thought to be at least partly responsible for the fact that matter outlives antimatter in our universe. Recently both the NA48 experiment at CERN and the KTeV experiment at Fermilab have directly measured this asymmetry with enough precision to establish it. And a number of experiments, including the BaBar experiment at the Stanford Linear Accelerator Center and Belle at KEK in Japan, will confront the same question in different particle systems.
Antimatter at lower energies is used in Positron Emission Tomography (see this PET image of the brain).
But antimatter has captured public interest mainly as fuel for the fictional starship Enterprise on Star Trek.
In fact, NASA is paying attention to antimatter as a possible fuel for interstellar propulsion. At Penn State
University, the Antimatter Space Propulsion group is addressing the challenge of using antimatter
annihilation as source of energy for propulsion. See you on Mars?
Answer originally posted October 18, 1999