HI gas Interstellar absorption lines Absorption lines of Hydrogen, Calcium, Sodium, Nitrogen, Oxygen, etc. seen.
Abundances are about the same as seen in the sun.
Radio emission from Neutral hydrogen. Produced by hydrogen atoms spontaneously changing from state where electron in orbit and proton in nuclei are spinning in the same direction to a state where the electron is spinning in one direction, and the proton in another. Because H is so abundant, we find this coming from all over the Milky Way. The 21 cm radiation, in fact, us used to map out the overall shape of the Milky Way galaxy we live in. Typical HI region has Temp around 100K and density around 10-50 atoms in each cubic centimeter.