Questions about material composed of antiparticles, which have the same mass as particles of ordinary matter but opposite charge and properties, such as lepton and baryon number.
For every particle there is an antiparticle, e.g. the positron as antiparticle to the electron. There are also antiatoms, for example antihydrogen which consists from an antiproton and a positron.
Particles and antiparticles differ only in that their charges (for example the electric charge) or more precisely their charge-like quantum numbers are opposite. While the electron is electrically negatively charged, the positron is correspondingly positively charged. Other properties such as mass are exactly the same (as far as known so far).
In 1928 the physicist Paul Dirac predicted for the first time the existence of antimatter from theoretical considerations, positrons were proved as first antiparticles then in 1932.