Antimatter or contra-terrene matter is matter that is composed of the antiparticles of those that constitute normal matter. If a particle and its antiparticle come in contact with each other, the two annihilate and produce a burst of energy, which results in the production of other particles and antiparticles or electromagnetic radiation. In these reactions, rest mass is not conserved, although (as in any other reaction) energy (E=mc²) is conserved.
Scientists in 1995 succeeded in producing antiatoms of hydrogen, and also antideuteron nuclei, made out of an antiproton and an antineutron, but no antiatom more complex than antideuterium has been created yet. In principle, antiatoms of any element could be built from readily available sources of antiparticles. Such antiatoms would have exactly the same properties as their normal-matter counterparts. The production of antielements in bulk quantities seems unlikely to ever become achievable, however.
Positrons and antiprotons can individually be stored in a device called a Penning trap, which uses a combination of magnetic field and electric fields to hold charged particles in a vacuum. Two international collaborations, ATRAP and ATHENA, used these devices to store thousands of slowly moving antihydrogen atoms in 2002. It is the goal of these collaborations to probe the energy level structure of antihydrogen to compare it with that of hydrogen as a test of the CPT theorem. One way to do this is to confine the antiatoms in an inhomogenous magnetic field (one cannot use electric fields since the antiatoms are neutral) and interrogate them with lasers. If the anti-atoms have too much kinetic energy they will be able to escape the magnetic trap, and it is therefore essential that the anti-atoms be produced with as little energy as possible. This is the key difference between the antihydrogen that ATRAP and ATHENA produced, which was made at very low temperatures, and the antihydrogen produced in 1995 which was moving at a speed close to the speed of light.
Antimatter/matter reactions have practical applications in medical imaging, such as positron emission tomography (PET). In some kinds of beta decay, a nuclide loses surplus positive charge by emitting a positron (in the same event, a proton becomes a neutron, and neutrinos are also given off). Nuclides with surplus positive charge are easily made in a cyclotron and are widely generated for medical use.
Antiparticles are created everywhere in the universe where high-energy particle collisions take place, such as in the center of our galaxy, but none has been detected that is residual from the Big Bang, as most normal matter is. The unequal distribution between matter and antimatter in the universe has long been a mystery. The solution likely lies in the violation of CP-symmetry by the laws of nature.