Interstellar gas clouds consisting mostly of neutral hydrogen atoms (commonly called HI, or H-one atoms by astronomers) are usually referred to as HI clouds. They are very abundant, but mainly confined to the spiral arms in the disks of galaxies in a layer less than 300 light years thick. They have an average temperature of about 100 Kelvin and densities that range from 1 - 100 atoms/cm3 spread over distances of between 15 and 20 light years.
Because they are cold, they do not emit radiation in the visible part of the spectrum. Rather, they are detected using the spin-flip transition at 21cm in the radio, and have been particularly important in mapping out the structure of our own Galaxy.
How to map the Galaxy from within
Radio waves are largely unaffected by dust, allowing us to detect HI clouds throughout our Galaxy. Each HI cloud along a particular line of sight will be moving at a slightly different speed relative to us, meaning that the 21cm radiation emitted by the cloud will be Doppler shifted by a different amount when it arrives at our telescopes.