A dark nebula is a large molecular cloud of interstellar nonluminous dust and gas that absorbs partly or entirely the light of background stars or the light of an emission or reflection nebula (i.e. the Horsehead Nebula).

These molecular clouds are among the best known cold objects (about 10 °K or -263 °C), and events such as the formation of stars and masers take place.

Horsehead Nebula

Other deep sky objects: