Dark Nebulae

Dark nebulae or absorption nebulae are interstellar or molecular clouds that are so dense that they obscure the visible wavelengths of light from objects behind it, such as background stars and emission or reflection nebulae. The extinction of the light is caused by interstellar dust grains in the coldest, densest parts these molecular clouds. Clusters and large complexes of dark nebulae are associated with Giant Molecular Clouds.  

Sometimes we see isolated small dark nebulae within areas of bright nebulae, these small dark patches are called Bok globules and are areas of dense cosmic dust and gas, in which star formation may take place. Known to be some of the coldest objects in the natural universe, their structure and density remain somewhat a mystery.