The Mordag Nebula is a dim stellar remnant, believed to have been created out of a two stars passing close together. Both stars would have been too far away to capture each other into a new binary star system. In theory when these two stars passed close enough the tidal forces between the stars ripped each other apart. One or both stars may have been a white dwarf given the heavy elements found in the nebula. While a very dense and cool nebula it is subject to wildly unpredictable but mildly strong shifts of gravity from subspace eddies and currents produced by the Pandoric Rift located within. The gravitational and subspace currents do create mild hot-spots with give off faint light, able to be seen as one approaches the nebula but not far beyond that.
The Nebula is about 118 ly from the Markab Star System and 267 ly from Earth. It is 7 ly across at its widest point and 5 ly deep similarly at its deepest point. The exact location of the Pandora rift is classified, neither the Federation nor the Vulcan Science Council known exactly where the rift is.
In addition to the Pandoric Rift and Starbase Pandora, there is a classified Jump Gate within the Nebula as well near Starbase Pandora.