a dense - on a cosmic scale - cloud of gas and dust, which is cold enough to allow more or less complex molecules to appear
assive clouds of gas in interstellar space composed primarily of hydrogen molecules (two hydrogen atoms bound together), though also containing other molecules observable by radio telescopes. These clouds can contain enough mass to make several million stars like our Sun and are often the sites of star formation.
Huge, cool clouds of dust grains, and gas, much of which is in the form of molecules. GMC's appear to be where most of the stars are formed in galaxies.