Anti-matter universe
Omega was trapped in and ruled over the anti-matter universe.
It was situated in a black hole, leaving it a pocket universe. Omega shaped this universe as he saw fit, creating his own paradise in which to dwell.
Over the centuries, however, he grew despondent due to his isolation. The landscape slowly became unstable because of this. Omega planned to escape the anti-matter universe and return to N-Space. However, living inside the universe had a corrosive effect, causing Omega's physical body to wear away. This made it impossible to leave the universe, because only his will allowed him to live, and keep it from collapsing. (TV: The Three Doctors, Arc of Infinity)
Eventually, Omega managed to escape the universe possessing the body of the Time Lord Tauras and trapping behind another Time Lord to sustain the anti-matter universe. (AUDIO: Intervention Earth)
Zeta Minor was a gateway to an anti-matter dimension. (TV: Planet of Evil)
