Dark nebula

     Image source: [1]

A dark nebula is a type of interstellar cloud so dense that it hides the light from the background stars. Fortunately some stars of the Milky Way come to pierce the vast dark clouds separating the plane of our galaxy. In all the nebulae there are dark clouds, they are also the ones that embellish the nebulae, the best example is the Horsehead Nebula or the Snake Nebula.

The extinction of light is caused by the interstellar grains of solid dust that make up the cloud. Large complexes of dark nebulae are associated with giant molecular clouds. Isolated small dark nebulae are called "Bok globules". Dark clouds appear thus, because of micrometric dust particles, coated with carbon oxide and frozen nitrogen, which effectively blocks the passage of light at visible wavelengths.

More details here: [2]

Dark Nebula Horse Head

     Image source: [3]

Nebula Horsehead is a dark nebula in the Orion constellation.

The nebula lies just below Alnitak (ζ Ori), the easternmost star of the Orion Belt. The nebula, located 1 350 light-years away, was discovered in 1888 on a photographic plate taken from the Harvard College Observatory. It is easily recognizable by the horse's head shape, which gave it its name. Horse Nebula is part of a large molecular black cloud.

More details here: [4]

The Dark River

     Image source: [5]

The central band of the Milky Way is connected to Antares by a path of dark dust called "Black River". It is a cosmic cloud that connects the Pipa Nebula to the colorful region located near the bright star Antares in the constellation Scorpius. The opacity is due to the absorption of light from the background stars by the cosmic dust. The nebula contains essentially hydrogen and molecular gas.

Nebula of the Pipa

     Image source: [6]

The Pipa nebula is a dark nebula in the constellation Ophiuchus. This cloud of gas and dust spreads over vast territory in the sky, and belongs to an even larger complex called the Dark Horse Nebula. Despite its darkness, observers can easily see the Pipa Nebula with the naked eye, far from the city lights.

More details here: [7]