The coldest regions of space are the dark nebula. Some dark nebula are found through star counts, such as the Coalsack. Others are visible because an emission nebula is behind them, so they are illuminated from behind, such as the Horse Head Nebula shown below. This happens often because the same gas that is associated with the dark nebula can also be heated by nearby stars to glow as an emission nebula. The Horse Head Nebula is several tens of parsecs across and would envelop the local neighborhood of stars around our solar system.
Stars seen in the image are foreground stars since the nebula is opaque.
Some dark nebula are surrounded by emission regions, giving the impression that the dark cores are being dissolved by the hotter gas as in the Eagle Nebula shown below. A great deal of information is contained in the above image, the irregular shape of the dark regions, the hot vapor off the edges heated by nearby stars, the straight rays of dissolved gas. This nebula indicates that the interaction between dark nebula and bright, hot gas is dynamic and ongoing. Often in the centers of dark nebula are dense concentrations of gas and dust called molecular clouds. Dense gas makes for instability to gravity, and large pieces of the cloud can collapse to form protostars.