When Dib's head temporarily opened a transdimensional portal, he had actually opened a wormhole into a parallel universe. This galaxy is identical to the one that Earth and Irk reside in, save for the fact that it's a dark, evil, shadowy version of our universe.
On the particular night that Dib had been experimenting with the dimensional scope Professor Membrane had made, when it glitched and created a portal in his head, it actually created a paradox in itself, a whole new universe. The inhabitants were created as an alternate, corrupted version of his universe in the accident, particularly influential from his actual imagination of his perception of the world around him, perhaps creating "monsters" of whom he actually knew. However, he had only stumbled into the Earth of that dimension, his home town. There exists an Irk and many other darker clones of planets from Z-14.
It is important to include Nightmare Irk in any description of the Nightmare dimension. This planet is the homeworld of the Nightmare Irkens and the base of operations for the Nightmare Irken empire, which has recently been making risky grabs for territory in its own dimension and in others, including Z-14, despite dealing with political and military fragmentation in its own chain of command. Political and military strategists in the Irken Empire suggest that it is only a matter of time before the whole Nightmare Irken regime collapses under its own weight.
In the mean time, though Nightmare Irk is a real and immediate threat to all surrounding universes, and having gained mastery of Interdimensional teleportation from a being called Dusq and the inventions of Nightmare Vex, the chief engineer in Nightmare Clod's army, they are now even more threatening. With the destruction of a Mind of Dusq on Nightmare Irk, the Nightmare Irken society has been plunged into even more chaos, and no one can currently predict the result.
The Nightmare Dimension is a strange alternate dimension which amplifies the negatitive aspects of Z-14 of it's counterparts to the point of creating the dark, horrible creatures that inhabit it. The inhabitants of the Nightmare dimension tend to be agressive and warlike, but some of them are not depending on their otherwordly counterpart. The Nightmare counterparts of humans typically bare little resemblence to their counterparts, with some exceptions.
Irken counterparts, however (dubbed "Nightmare Irkens") usually bare a rough, yet distinct resemblence to normal Irkens. The Nightmare equivalent of the Irken empire is even more expansionist, and almost always eventually betrays the few allies it makes once they are no longer found useful.
- It is implied the Nightmare Dimension has different physical laws then the prime dimension.