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.
A list of theoretical particles
Particles that disobey laws of physics
Particles that require an incredible amount of energy to create
Other hypothetical particles
Theoretical particle Facts for Kids. Kiddle Encyclopedia.