Astronomers have discovered two massive clouds of hydrogen gas near the Whirlpool galaxy, also known as M51, that contain nearly no stars. These clouds, identified using China's FAST radio telescope, each hold about 3 million times the mass of the Sun in hydrogen but show no signs of stars when observed with optical telescopes. The findings were published in the journal Astronomy & Astrophysics on August 4. In the standard model of the universe, known as the Lambda Cold Dark Matter (LCDM) model, structures like galaxies are thought to form in a hierarchical way. Small dark-matter halos form first and then merge to create larger ones, eventually leading to galaxies. However, not every dark-matter halo becomes a galaxy. Some may not form stars due to various factors, such as weak gravitational pull or gas that is too warm or diffuse to collapse into stars. The ultraviolet (UV) background, a form of radiation from distant stars and quasars, can also heat gas, preventing it from cooling and forming stars. In a new study led by Qingze Chen from the National Astronomical Observatories of the Chinese Academy of Sciences, researchers looked for hydrogen clouds that might be remnants of dark-matter halos that retained gas after a period in the early universe known as reionization. These clouds, called reionization-limited H I clouds (RELHICs), are expected to be compact, roughly spherical, and have hydrogen gas that is not cold enough to form stars. To qualify as a RELHIC, the cloud must show narrow spectral lines and have no detectable stars. The researchers used the Five-hundred-meter Aperture Spherical Telescope (FAST), also known as the "China Sky Eye," to detect 21-cm radio signals from neutral hydrogen. They analyzed data from 55 galaxies in the FEASTS survey and found only two clouds—Cloud N and Cloud S—that met the criteria. Each has about 3 million solar masses of hydrogen and a total dark-matter halo mass of 3.7 billion solar masses, matching theoretical predictions. Both clouds are located around 70–90 kiloparsecs (about 228,000–293,000 light-years) from the center of M51, a galaxy known for its active interactions with a smaller companion galaxy. While the researchers cannot rule out the possibility that these clouds are debris from a galactic collision, further high-resolution observations are needed to confirm whether they are indeed starless dark-matter halos or remnants of a recent galactic interaction.