Dark space ambient (aka black space ambient) is basically space-themed dark ambient, featuring echoing, dark synths, bubbling effects, incorporating cosmic ambiences like signals from celestial objects, noisy transmissions from spacecrafts, etc.
Even though it share similarities with space ambient (which usually features flowing and relaxing sounds), the atmosphere created by dark space ambient artists is significantly different. Makes you feel as if you’re on a desolate spaceship orbiting a planet in the darkest region of space. At other times sounds as if you are locked inside the engine room of an UFO.