Antimatter is matter composed of antiparticles, such as positrons, antineutrons and antiprotons. For each type of ordinary particle (e.g., electrons or protons), there is an antiparticle. Antiparticles have opposite charges and certain other quantum properties of opposite sign.

Collectively, antiparticles with mass, and atoms composed of them, are referred to as antimatter. Particles and antiparticles have the same mass and spin. When a particle and antiparticle are brought together they annihilate each other, converting all of their mass and energy into a burst of electromagnetic radiation, and sometimes into other particles. Some neutral particles such as photons are their own antiparticles, while others like neutrons have antiparticles with opposite magnetic moments.

In general, the distinction between matter and antimatter is somewhat arbitrary. With protons, neutrons, electrons, and their antiparticles, the matter-antimatter distinction is based on the fact that all of the ordinary objects we encounter are composed...