The micro-world of atoms and particles is still a recent discovery when viewed historically. The electron was discovered in 1897 by J. J. Thomson; the proton in 1911 by Rutherford, Wein, et al., the neutron in 1932 by James Chadwick, and the positron was first theoretically identified in 1928 by Paul Dirac.

The American physicist Anderson (1905- 1991) confirmed Dirac’s anti-matter (positron) theory. In 1927, Anderson had begun studying X-ray photoelectrons (electrons ejected from atoms by interaction with high-energy photons). In 1932, Anderson experimentally confirmed the existence of the anti-matter positron.  More here