Paul Dirac, based on the Pauli Exclusion Principle, proposed that the vacuum itself is saturated with particles occupying all possible negative-energy states; in practice, the vacuum is a huge inert atom that prevents (excludes) electrons from falling into increasingly negative-energy states (a solution possible with theoretical formulas and which had until then caused physicists no small headaches); the absence of a particle in a negative-energy state (for example, if it is given enough energy to emerge from the sea of negative energy) corresponds to the presence of positive energy E=mc2 where m is a positron (the electron’s antimatter), which would actually be discovered 6 years later, in 1932.
