Antimatter is material with properties equivalent to those of matter but with the quantum properties of the constituent subatomic particles reversed; for example, the particles' charges are opposite. Antimatter is composed of antiparticles. (Note that both the mass and energy of antimatter is positive not negative. (See negative mass.) Thus, antimatter does not give rise to antigravity.
Antimatter happens to be rare in our universe. However, it is believed that in the Big Bang equal quantities of matter and antimatter were created. The fact that the universe now contains matter and not antimatter is known as the matter-antimatter asymmetry. To understand how this asymmetry came about is a major goal of contemporary particle physics and astrophysics. An experiment called the Alpha Magnetic Spectrometer, due to launch to the International Space Station in May 2011, is designed to shed new light on this issue.
Antimatter can be created briefly in laboratory high-energy collisions. The heaviest pieces of antimatter ever formed artificially are nuclei of anti-helium, each containing two anti-protons and two anti-neutrons. They were created and detected at the Relativistic Heavy Ion Collider (RHIC) in Upton, New York, in 2011. The next heaviest anti-element, anti-lithium, could in theory form solid antimatter at room temperature, but will be much harder to make.