Antimatter, matter composed of antiparticles (subatomic particles with certain properties, such as electrical and magnetic properties, opposite those of ordinary subatomic particles with the same mass). Antiparticles exist for each type of subatomic particle known. The antiparticles of protons, neutrons, and electronsthe particles of which atoms are madeare called antiprotons, antineutrons, and positrons (antielectrons). Protons and neutrons are composed of smaller particles called quarks; antiprotons and antineutrons are composed of antiquarks, the antiparticles of quarks. Antiparticles carry the electric charge opposite to that of their particles and spin differently but have the same mass and resemble them in all other ways. It is possible that stars or even galaxies of antimatter exist in other parts of the universe. An antimatter galaxy would look exactly like a galaxy of ordinary matter. Some scientists think the universe may contain as much antimatter as ordinary matter. However, naturally occurring antimatter is apparently absent in the universe. Most astronomers believe that there is relatively little antimatter, because otherwise, they reason, there would be extensive regions where ordinary matter and antimatter meet, releasing a large amount of gamma rays as they undergo annihilation. Annihilation is the destruction of both ordinary particles (matter) and antiparticles (antimatter) following their collision, which is followed by a release of energy. No such regions have been found. Many scientists conjecture that the universe began with an explosioncalled the big bangthat occurred when all the matter collided with all the antimatter and that there was a little matter left after the annihilation, which is all the matter in the universe today. Antiparticles occur in certain kinds of radioactive decay. Antiparticles also occur in cosmic rays and among the particles produced when cosmic rays collide with the atoms in the earth's atmosphere. Physicists who study particle physics produce antiparticles and use them in machines called particle accelerators to learn more about the nature of matter. In certain storage-ring particle accelerators, artificially produced antiparticles can be maintained for many days by circulating them in a ring-shaped vacuum chamber. Antiparticles combine in the same manner as ordinary particles. For example, an antineutron may combine with an antiproton to form an anti-deuteron, the nucleus of an anti-deuterium atom. Antiparticles can also combine with ordinary particles. For example, an electron may combine with a positron to form a positronium, an unstable atom.
The British theoretical physicist Paul A. M. Dirac predicted the existence of the antiparticle positron in 1930, during the course of developing equations to describe the properties and behavior of the electron. In 1932, the United States physicist Carl Anderson discovered the positronthe first discovery of an antiparticlewhile conducting research on cosmic rays. In 1955, the antiproton was discovered by Emilio Segr (an Italian physicist who emigrated to the United States), Owen Chamberlain (a United States physicist), and their co-workers. They produced antiprotons by bombarding a copper target with high-speed protons from a particle accelerator. The antineutron was discovered the following year by a similar technique. Since then, many other antiparticles have been discovered in experiments conducted with particle accelerators. In 1996, physicists at the CERN laboratory near Geneva, Switzerland, created antihydrogen, each atom of which consisted of a single antiproton as nucleus and a positron revolving round it. If the techniques used at CERN are developed further, scientists might be able to compare hydrogen atoms with antihydrogen atoms in detail.
In 2007, scientists bombarded a thin, porous film with positrons and created positronium molecules by fusing two positronium atoms.