The existence of antiparticles was predicted by Dirac a few years before the first one, the antielectron or positron, was found. The idea stemmed from the existence of negative energy states, which in a relativistic universe can not be discarded a priori. Since electrons normally seek the lowest possible energy state, Dirac posited that these extra states must all be filled with what are called [virtual particle]?s. In that case, a virtual particle could be promoted to a positive energy state, creating a real particle and leaving a hole that would behave exactly the same, but with opposite charge.