Little is known of this world, save that it is frequented by Shadows, and lies far out on the Rim.
Morden, a human who was aboard the science vessel Icarus when it landed on Z'ha'dum, seems to act as the Shadows Babylon 5 contact.
Shadow vessels have never been observed to have visible means of propulsion, so they may be assumed to have non-Newtonian drives, and by extension, artificial gravity. There are indications that they also utilize organic technology on a level similar to the Vorlons.
The Shadows are incredibly old beings. They have the power of limited invisibility, but it appears that they cannot be invisible on all wavelengths at once, nor can they escape psychic detection.