A kind of theoretical particle said to exist by some theories of modern physics, whose alleged properties are extremely unusual. Examples include tachyons (particles that always travels faster than the speed of light), WIMPs (weakly interacting massive particles which do not interact with electromagnetism or the strong nuclear force), axions (particles with no electric charge, very small mass and very low interaction with the strong and weak forces) and neutrinos (particles that travel close to the speed of light, lack an electric charge and are able to pass through ordinary matter almost undisturbed).