In some galaxies, the interstellar gases are glowing brightly. This happens in galaxies with lots of star formation. Stars form in cocoons of gas and dust. Newborn stars illuminate the surrounding gas with ultraviolet radiation. This UV radiation ionizes the gas.
Ionized gas glows brightly as the atoms recombine.
The glow occurs at special wavelengths also. These show up as emission lines in the galaxy spectra. I'm now going to show you a few galaxy images and spectra. If you're into exploring the Web yourself, however, follow this link to access the full selection. For example, here's a galaxy, NGC 2276, that exhibits pretty much a pure emission line spectrum.