Particles with certain properties opposite to those of matter. Each matter particle has a corresponding antiparticle. The antiparticle has exactly the same mass and the opposite electric charge as its partner. An example is the electron (negative charge) and its antimatter version the positron (positive charge).
When a particle and its antiparticle collide, both are annihilated and converted into photons. Similarly two photons with sufficient energy can combine to form a particle-antiparticle pair.
The universe is made almost entirely out of matter. This means that in the big bang there was an excess of matter over antimatter so that when matter and antimatter combined and annihilated, some matter was left over. (See also baryogenesis.)