in astronomy, interstellar matter composed of neutral hydrogen atoms. Most of the matter between the stars in the Milky Way Galaxy, to which the Earth belongs, as well as in other spiral galaxies, occurs in the form of relatively cold, neutral hydrogen gas with a temperature of only about 100 K and a density of approximately one atom per cubic cm. There also exist cold compact hydrogen clouds with a density of at least 10 particles per cubic cm. Hydrogen clouds are easily detectable at radio wavelengths because they emit a characteristic energy at a wavelength of 21 cm (see 21-centimetre radiation).