Antimatter was first predicted theoretically by the British physicist Paul Dirac in 1928, and first observed experimentally by the American physicist Carl D. Anderson in 1932.
Prediction by Dirac
Paul Dirac developed a relativistic equation for the electron that implied the existence of particles with the same mass as the electron but opposite electric charge. This work introduced the concept of an “antielectron,” later called the positron, and marks the theoretical discovery of antimatter.
Experimental discovery by Anderson
Carl D. Anderson discovered the positron while studying cosmic rays with a cloud chamber in 1932. His observation of a positively charged electron track provided the first direct evidence of antimatter in nature and earned him the Nobel Prize in Physics in 1936.