Dimension X was home of several characters who often create difficulties for the Turtles on Earth, most notably one of the main antagonists the evil alien warlord Krang in the Archie Comics. Characters usually use the Trans-Dimensional Portal l located inside the Technodrome for transportation between Dimension X and the Earth, though other portals and other means of travel are occasionally seen.
It is not explained where this place is and whether it really is a parallel dimension, as the name suggests, or possibly an another galaxy; in the comic series, both options are applied, but a definitive determination is never revealed.
In any case, it is at Dimension X is the spiral galaxy in which there are numerous inhabited (or habitable) worlds. In contrast to the earthly space, space is colored in Dimension X reddish-orange and various forms of life to guarantee able to survive without any protection or breathing equipment.