Dark nebulae are cold and dense molecular clouds that appear as star-poor regions in the sky. These dark clouds are irregular in shape and contain much of the mass of the interstellar medium—that is, the stuff between the stars. These clouds are the place where new stars are formed.

The largest dark nebulae are visible to the naked eye as the gaps in space against the brighter background of the Milky Way. A good example of a dark nebula is the Coalsack Nebula.