Astronomers using China’s FAST radio telescope have discovered a compact cloud of hydrogen with no visible stars or galaxy around it. This could be one of the strongest candidates yet for an isolated “dark galaxy,” containing nearly 500,000 times the sun’s mass in neutral hydrogen. The discovery paper was posted to the arXiv preprint server on Sept. 24.

In the lambda cold dark matter (ΛCDM) model, structure forms hierarchically: small dark matter halos form first and then merge to build larger halos and galaxies. The model also predicts that some low-mass halos never form stars at all.

Several processes can suppress star formation at low mass. One is that, after cosmic reionization, the halo’s gas is heated by the ultraviolet (UV) background (the diffuse radiation from quasars and galaxies) and so never becomes cold and dense enough to form stars. Such starless, gas-bearing halos are predicted to exist and are called reionization-limited H I clouds (RELHICs) or “dark galaxies.”