Dark Nebulae

Dark nebulae are clouds of dust which are simply blocking the light from whatever is behind. Dark nebulae are also often seen in conjunction with reflection and emission nebulae. They are very similar to reflection nebulae in composition and look different primarily because of the placement of the light source. Dark nebulae are not seen by their emitted or reflected light. Instead, they are seen as dark clouds in front of more distant stars or in front of emission nebulae.
The Horsehead Nebula in Orion is probably the most famous example of a dark nebula. It is a dark region of dust in the shape of a horse's head that blocks the light from a much larger emission nebula behind it.

The Horsehead Nebula