if im not mistaken, there are a lot of things that have never been seen that have been theoriezed about, but that have not been disproven.

http://scienceworld.wolfram.com/physics/Tachyon.html
http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/tachyons.html
http://en.wikipedia.org/wiki/Tachyon

(this last one includes the theory that they can go backwards in time.)

there is also the theory that some neutrinos are actually tachyons

no, i have not seen any, but niether have i ever seen a proton or electron, yet i dont see anyone argueing that they dont exist.

the more man learns, the more he realises, he really does not know anything.