Dust grains serve as sites for the formation of molecules and organic compounds. Their cold surfaces act as catalysts by allowing atoms to stick to them so there there is time for a second atom to land, interact, and form a molecule. Collections of dust and molecular gases are called molecular clouds. Hot regions of interstellar gas are bright in their own optical emission. Note also that the various temperatures determine the type of matter that will exist.
Cold temperatures are suitable to the formation of molecules.
Warmer temperatures will find only atoms, such as neutral hydrogen. 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.