Theoretical particles are particles that have been assumed or predicted to exist by scientists, but have not been proven in any experiment. Some, like the tachyon, probably don't exist, since they violate several laws of physics. However, many are still believed to exist. All supersymmetric particles (such as a sfermion) are theoretical. Supersymmetrical particles are often abbreviated with an "s" in front of the particle name, like a sfermion. Particles found in antimatter are not theoretical particles because they "have" been found in numerous experiments.