OK, so "No Light, No Light" isn't entirely accurate. There is light in this setting, but not in the dangerous places...and it seems like 99% of the world is dangerous places.
I built up a rough 3D form for the overall setting. The image above is a rendering on the from outside, showing that it basically exists as a series of concentric shells. The image below is a rendering done with the lighting expected between the two of the shell layers.
Gravitationally, this is interesting, because the force of gravity on the outside of a shell is equivalent to the gravity that might be exerted from a point mass of equivalent weight (see here), while the gravity within a shell is in equilibrium (see here).
Mathematically that's just wierd, especially when we consider shells within shells.
The innermost section (A) would exist within all three shells. Therefore gravity here would be in total equilibrium. The inner habitable section (B) would exist outside the inner shell (and thus be effected by it's gravity), but inside the mid and outer shells (and thus not affected by their gravity fields). The outer habitable section (C) exists outside the inner and mid shells (and is thus affected by their gravity fields), but not the outer shell. Outside the entire structure (D) is affected by the gravity of all three.
I was worried about the variable gravity at various points in the construct, but this might not be an issue after all. As long as I make the inner sphere (50 km radius) a triple thickness, the middle sphere (75km radius) a double thickness, and the outer sphere (100km radius) a single thickness, then the gravitational ratios remain relatively constant across the layers.
The actual theory here is to make the shells an intricate network of tunnels, chambers and passages, these are the locations where exploration will occur and where mysteries will be revealed. Open area "A" is directly exposed to whatever lies at the centre of this structure, the inner shell makes sense to be triple thickness to protect open area "B". Open area "B" still has higher than average radiation from the inner anomaly, and possibly limited astral/hyperspace radiation. The mid shell (between "B" and "C") would be where most of the inhabitants exist, living on the "surface" of that sphere and in the tunnels that riddle it. Open area "C" has minimal radiation from the inner anomaly, but more intense astral/hyperspace radiation from outside. Nothing much exists on the surface of the outer shell expect for astral/hyperspace vessels which have accidentally crashed into the structure (thus becoming the inhabitants of the setting). Some vessels may have crashed through the outer shell and pierced into open area "C" but these are rare, and over decades, the structure's automated repair systems have sealed the holes that would have been caused.
The shells are basically made up of geodesic forms, the nets of these forms are similar to soccer balls where the hexagons are riddled with tunnels and habitable structures, while the pentagons are uninhabitable and filled with the mechanisms necessary for structural integrity. We'll get a bit more specific with maps later...not too specific, that's what random tables are for during the course of play, but certainly more specific than we've got now.
There is no sun that this structure orbits, there is no natural light unless you count the fires set by wandering adventurers. This is a generally a place of darkness and flickering shadow, those few locations where mystical gems and fungal blooms light up the passageways and chambers are highly sought. They may not be safe, but at least they let you see what you're fighting.
Enough mathematics and physics, the world as been built. Now it's time to start populating and detailing it.