A dark nebula is a type of interstellar cloud

Interstellar cloud

Interstellar cloud is the generic name given to an accumulation of gas, plasma and dust in our and other galaxies. Put differently, an interstellar cloud is a denser-than-average region of the interstellar medium. Depending on the density, size and temperature of a given cloud, the hydrogen in it...

 that is so dense that it obscures the light from the background emission

Emission nebula

An emission nebula is a cloud of ionized gas emitting light of various colors. The most common source of ionization is high-energy photons emitted from a nearby hot star...

 or reflection nebula

Reflection nebula

In Astronomy, reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars. The energy from the nearby star, or stars, is insufficient to ionize the gas of the nebula to create an emission nebula, but is enough to give sufficient scattering to make the dust...

 (e.g., the Horsehead Nebula

Horsehead Nebula

The Horsehead Nebula is a dark nebula in the constellation Orion. The nebula is located just to the south of the star Alnitak, which is farthest east on Orion's Belt, and is part of the much larger Orion Molecular Cloud Complex. The Horsehead Nebula is approximately 1500 light years from Earth...

) or that it blocks out background stars (e.g., the Snake Nebula

Snake Nebula

The Snake Nebula is a dark nebula in the Ophiuchus constellation. It is a small but readily apparent S-shaped dust lane that snakes out in front of the Milky Way star clouds from the north-north-west edge of the bowl of the Pipe Nebula. Its thickness runs between 2′ and 3′ and runs...

). The extinction

Extinction (astronomy)

Extinction is a term used in astronomy to describe the absorption and scattering of electromagnetic radiation by matter between an emitting astronomical object and the observer. Interstellar extinction—also called Galactic extinction, when it occurs in the Milky Way—was first...

 of the light is caused by interstellar dust

Cosmic dust

Cosmic dust is a type of dust composed of particles in space which are a few molecules to 0.1 µm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust .In our own Solar...

 grains located in the coldest, densest parts of larger molecular cloud

Molecular cloud

A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen ....

s. Clusters and large complexes of dark nebulae are associated with Giant Molecular Clouds. Isolated small dark nebulae are called Bok globules.