The phrase "hydrogen clouds" refers to vast collections of hydrogen gas that float in space. These clouds are made of hydrogen molecules and are so big that they can stretch across galaxies. Full definition
By 2005, however, optical telescope follow - ups on these sources suggested they were almost all just hydrogen clouds in normal galaxies. (discovermagazine.com)
In the spectrum, the team found evidence of a large concentration of neutral hydrogen clouds close to the galaxy, indicating the presence of a giant cluster of embryonic galaxies. (sciencemag.org)
To gauge 3C 58's distance, astronomers exploit the Milky Way's rotation, measuring velocities of hydrogen clouds in front of the nebula to deduce how fast it revolves around the galaxy's center. (scientificamerican.com)