An interstellar cloud made up of gas, plasma and galaxy dust can sometimes obscure light from background stars or a reflection nebula creating irregular shapes that have no defined boundaries. As a result, a dark nebula, or absorption nebula, is formed. Large dark nebula can be seen by the naked eye, often appearing as dark patches scattered in the sky.

Inside the dark nebula, an important even occurs – the formation of stars and masers. Due to the density difference, star formation is only possible during condensation. The larger dark nebula is often associated with giant molecular clouds while the smaller ones are called Bok globules. Half of the known Bok globules have been found to contain newly formed stars. There are different kinds of Dark Nebula, the most famous being the Horsehead Nebula. It is also known as Barnard 33 and is found in the constellation of Orion. Despite its popularity, the Horse head Nebula is not easy to spot. On the other hand, there is the Coal sack Nebula which is easier to find. It is located in the constellation of Crux and is only found in the skies of the Southern Hemisphere. Another famous dark nebula is the Snake nebula that is located in the constellation Ophiuchus.