Clouds in space are made mostly of hydrogen gas.

Hydrogen cloud, also called H I region or H i region, interstellar matter in which hydrogen is mostly neutral, rather than ionized or molecular. Most of the matter between the stars in the Milky Way Galaxy, as well as in other spiral galaxies, occurs in the form of relatively cold neutral hydrogen gas. Neutral hydrogen clouds are easily detectable at radio wavelengths because they emit a characteristic energy at a wavelength of 21 cm.