In the night sky, the expanses of space between the stars of the Milky Way appear to be empty. In fact this space is occupied by a very thin gas that is mostly hydrogen and that has mere traces (less than 0.1% by number of atoms) of other elements such as oxygen, carbon, and nitrogen. The gas is also dusty; it contains grains of dust (particulate matter) that, like an interstellar fog, impede one's view of the stars. This gas is not evenly spread in space, but is clumpy. Although on average there is approximately one hydrogen atom for every cubic centimeter of interstellar space, a clump may be one thousand or more times as dense as a comparable volume of average density. Since about 1970 astronomers have been finding that these denser regions contain a great variety of molecules; about 120 different molecular species have been identified in the interstellar medium. The study of these molecules in the Milky Way and in other galaxies is called astrochemistry.