Anti-matter universe

The anti-matter universe was the universe that the Time Lord Omega became trapped in and subsequently ruled over.

The universe was situated in a black hole, leaving it a pocket universe. Omega was able to shape this universe as he saw fit, creating his own paradise in which to dwell. Although, over the centuries, he would grow despondent due to his isolation, and the landscape slowly became unstable due to this. Omega would then set out on a plan to escape the anti-matter universe and return to his original reality. The universe entailed a corrosive effect causing Omega's physical body to be worn away. (DW: The Three Doctors, Arc of Infinity)