Nullspace does not exist in the material universe. It is not an isolated dimension or a demesne, or part of our reality. It borders just beyond the infinity of the Astral Plane; a cold and empty reality opposite the burning energy of Hyperspace. It has been called many things: the Negative Zone; the Anti-Matter Universe; the Dark Dimension; and the Phantom Zone. It is one of the Fundamental Realms, the bedrock of all the existing realms in the Multiverse. Notably, this realm is where there are no walls between the realms of the Multiverse. There is only one Negative Zone, the drainage sump under the sink of the multiverse where all thought and matter and energy is destroyed.
Nothing in Nullspace makes sense to a mortal mind. Even gods struggle to understand it. Time does not flow in a linear fashion. Rules of natural causality are casually broken and reassembled on a whim. Whereas 'our' reality is constantly expanding, the Realm Between Worlds is constantly fading in enthalpic decay.
It is a broken and nightmarish place, filled with strange architecture and stars in the sky that are living, tortured beings. Life persists in Nullspace; notably the vast empire of Qward, home of the deposed Mobius.
There are other, far more terrible creatures to be found in this land: worst among them are the Old Gods. Fortunately they drift through infinity in fitful rest, a threat only to those foolhardy enough to wake them.