Antimatter is a rare, separate type of matter formed of Particles with some of their Quantum Numbers the opposite of those in corresponding normal matter, among these differing Quantum numbers being electrical charge. When a normal matter particle encounters its antimatter analog particle, they annihilate each other (known as Particle-Antiparticle Annihilation, or just Annihilation) the energy converted to other particles, generally Photons.
Antimatter is extremely rare locally and it is assumed that the observed universe is basically normal matter, i.e., there exists a Matter-Antimatter Asymmetry in the universe. The annihilation reaction of matter and antimatter together produces extremely high energy photons, and there are sensors to detect them. Cosmic Rays, which appear to come from distant parts of the universe, do not produce enough such photons when striking the Earth's Atmosphere to suggest anywhere near an equal balance among the sources of the particles, nor are such photons directly from distant sources sensed in numbers that would indicate many incidents of matter and antimatter coming together in such distant locations. There is no consensus as to this asymmetry's cause.
Antimatter particles include the Positron (i.e., Antielectron, like an electron but positively charged), the Antiproton, the Antineutron, Antineutrinos, and Antiquarks. The particles can be combined to form "anti atoms", e.g., Antihydrogen, and this has been done experimentally.
In minute quantities, positrons and antineutrinos are commonly produced by Beta Decay (thus in Nucleosynthesis) and other particles are commonly formed by high-energy collisions, e.g., in particle accelerators, and by cosmic rays striking ordinary matter. The particles are short-lived when surrounded by normal matter.