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Dark nebulae are opaque dark clouds in the form of molecular hydrogen without clearly defined outer boundaries. The largest nebulae of this type, the so-called giant molecular clouds (GMC), are more than a million times as massive as the Sun. They contain much of the mass of the interstellar medium, are some 150 light-years across, and have an average density of 100 to 300 molecules per cubic centimetre and an internal temperature of only 7 to 15 K.

Molecular clouds consist mainly of gas and dust but contain many stars as well. The cloud cores are completely hidden from view and would be undetectable except for the microwave emissions from their constituent molecules. GMCs play an important role in the Galaxy dynamics: when a star passes near a GMC, the considerable gravity pull will perturb the star's orbit by a significant amount. The largest dark nebulae are visible to the naked eye, appearing as dark patches against the brighter background of the Milky Way.