The exsistence of antimatter was posited by the electron theory of P.A.M. Dirac.  In 1932 the positron (antielectron) was detected in cosmic rays, this was followd by the antiproton and the antineutron.  All of these were detected through the use of particle accelerators.  Positrons, antiprotons, and antineutrons collectively called antiparticles, are the antiparticles of electrons and neutrons, respectively.  When matter and antimatter are in close proximity, annihilation (the combining of the two particle and them disappearing) within a fraction of a second, releasing LARGE amounts of energy!!!