Every type of particle in the universe has a corresponding anti-particle that has the opposite charge. The anti-particle of the negatively charged electron has a positive charge and is called the positron, while the anti-particles of the proton and neutron are the anti-proton and anti-neutron, respectively. The anti-proton has a negative charge (opposite the proton’s positive charge), and the anti-neutron is neutral, since the opposite charge of a neutral particle (no charge) is also neutral. Predicted in 1928 by physicist Paul Dirac, anti-particles were first detected in 1932.

       The early universe had nearly equal amounts of matter and antimatter, with just a slight excess of matter—about one extra particle for every 100 million photons and particle/anti-particle pairs. Because matter and antimatter annihilate one another in a burst of electromagnetic radiation (energy in the form of particles called photons, visible light is a kind of electromagnetic radiation) the universe we see today is dominated by the extra matter that couldn’t find antimatter with which to annihilate.