Paul Dirac published a paper mathematically predicting the existence of an antielectron that would have the same mass as an electron but the opposite charge. The two particles would mutually annihilate upon interaction.

“This new development requires no change whatever in the formalism when expressed in terms of abstract symbols denoting states and observables, but is merely a generalization of the possibilities of representation of these abstract symbols by wave functions and matrices. Under these circumstances one would be surprised if Nature had made no use of it,” he wrote.

Timeline:

Discovering the positron