What is a Dark Nebula?

Dark nebula are actually very similar to reflection nebula. The difference is where our line of sight is with respect to the dust and the energy source.  If the dust is between us and the energy source it simply absorbs and blocks the light and it’s a dark nebula. If our line of sight is on an angle to the dust and energy source then some of the light is reflected our way and it’s a reflection nebula. 

Examples of dark nebula include the Snake Nebula (Barnard 72) and the Horsehead nebula (Barnard 33).