Astronomers have confirmed the existence of a unique astronomical object, dubbed a Reionization-Limited H I Cloud (RELHIC), which appears to be a remnant from the early universe. This discovery challenges conventional understanding of galaxy formation and suggests the existence of previously undetected dark matter structures.
The Discovery of Cloud-9
The object, designated Cloud-9, was initially identified using the Five-hundred-meter Aperture Spherical Telescope (FAST) and later confirmed by the Very Large Array (VLA) and Green Bank Telescope (GBT). Located approximately 14.3 million light-years from Earth, near the spiral galaxy Messier 94, Cloud-9 exhibits the same recession velocity as its galactic neighbor.
What sets Cloud-9 apart is its composition: it is a starless cloud of neutral hydrogen dominated by dark matter. According to Dr. Alejandro Benitez-Llambay of Milano-Bicocca University, “This is a tale of a failed galaxy… seeing no stars is what proves the theory right.” This absence of stars supports the idea that Cloud-9 represents a primordial building block of a galaxy that never fully formed.
Why RELHICs Matter
The existence of RELHICs provides crucial support for the ΛCDM (Lambda Cold Dark Matter) model, a cornerstone of modern cosmology. This model predicts the existence of gas-filled, starless dark matter halos on sub-galactic scales that never ignited star formation.
The significance is clear: most of the universe’s mass is thought to be dark matter, which is notoriously difficult to detect because it does not emit light. Cloud-9 offers a rare opportunity to observe a dark matter-dominated cloud directly, providing empirical evidence for theoretical predictions. As Dr. Andrew Fox of AURA/STScI explains, “This cloud is a window into the dark Universe.”
What Hubble Revealed
The astronomers used Hubble’s Advanced Camera for Surveys (ACS) to search for any detectable stellar component within Cloud-9. Their analysis definitively ruled out the presence of even a dwarf galaxy with detectable stellar mass. This reinforces the interpretation that Cloud-9 is a true RELHIC: a starless dark matter halo in thermal equilibrium with the cosmic ultraviolet background.
The cloud itself spans roughly 4,900 light-years in diameter, making it one of the most compact HI clouds known to date. Its discovery also helps refine the understanding of the minimum mass required for dark matter halos to host galaxies, shedding light on why some halos form stars while others remain barren.
The discovery of Cloud-9 is a rare opportunity to study an early universe relic, advancing the field of cosmology toward a clearer understanding of dark matter and galaxy formation.
The findings were published this week in the Astrophysical Journal Letters (Anand et al., 2025). This discovery offers a critical step forward in understanding the distribution of dark matter and the conditions that lead to galaxy formation in the early universe.
