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Observations with the Five-hundred-meter Aperture Spherical Telescope have revealed the presence of a marginally resolved source of 21 cm emission from a location $\sim 50^{\prime} $ from the M94 galaxy, without a stellar counterpart down to the surface brightness limit of the DESI Imaging Legacy Survey (∼29.15 mag arcsec ^−2 in the g band). The system (hereafter Cloud-9) has round column density isocontours and a line width consistent with thermal broadening from gas at T ∼ 2 × 10 ^4 K. These properties are unlike those of previously detected dark H i clouds and similar to the expected properties of REionization-Limited-H i Clouds (RELHICs), namely, starless dark matter (DM) halos filled with gas in hydrostatic equilibrium and in thermal equilibrium with the cosmic ultraviolet background. At the distance of M94, d ∼ 4.7 Mpc, we find that Cloud-9 is consistent with being a RELHIC inhabiting a Navarro–Frenk–White (NFW) DM halo of mass M _200 ∼ 5 × 10 ^9 M _⊙ and concentration c _NFW ∼ 13. Although the agreement between the model and observations is good, Cloud-9 appears to be slightly, but systematically, more extended than expected for ΛCDM RELHICs. This may imply either that Cloud-9 is much closer than implied by its recessional velocity, v _CL9 ∼ 300 km s ^−1 , or that its halo density profile is flatter than NFW, with a DM mass deficit greater than a factor of 10 at radii r ≲ 1 kpc. Further observations may aid in constraining these scenarios better and help elucidate whether Cloud-9 is the first ever observed RELHIC, a cornerstone prediction of the ΛCDM model on the smallest scales.

Observations with the Five-hundred-meter Aperture Spherical Telescope recently detected H i 21 cm emission near M94, revealing an intriguing object, Cloud-9, without an optical counterpart. Subsequent analysis suggests that Cloud-9 is consistent with a gas-rich ( M _H I ≈ 10 ^6 M _⊙ ), starless, dark matter (DM) halo of mass M _200 ≈ 5 × 10 ^9 M _⊙ . Using the Karl G. Jansky Very Large Array in D-array configuration, we present interferometric observations of Cloud-9, revealing it as a dynamically cold ( W _50 ≈ 12 km s ^−1 ), nonrotating, and spatially asymmetric system, exhibiting gas compression on one side and a tail-like structure toward the other—features likely originating from ram pressure. Our observations suggest Cloud-9 is consistent with a starless ΛCDM DM halo if the gas is largely isothermal. If interpreted as a faint dwarf, Cloud-9 is similar to Leo T, a nearby gas-rich galaxy that would fall below current optical detection limits at Cloud-9's distance ( d ≈ 5 Mpc). Further observations with the Hubble Space Telescope reaching magnitudes m _g ≈ 30 would help identify such a galaxy or dramatically lower the current limits on its stellar mass ( M _gal ≲ 10 ^5 M _⊙ ). Cloud-9 thus stands as the firmest starless DM halo candidate to date or the faintest galaxy known at its distance.