  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 $\sim50'$ from the M94 galaxy, without a stellar counterpart down to
the surface brightness limit of the DESI Imaging Legacy Survey ($\sim29.15$ 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 $T\sim2\times10^4$ $K$. These properties are unlike those of previously
detected dark HI clouds and similar to the expected properties of
REionization-Limited-HI Cloud (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, $d\sim4.7$ Mpc,
we find that Cloud-9 is consistent with being a RELHIC inhabiting a
Navarro-Frenk-White (NFW) DM halo of mass, $M_{200}\sim5\times10^{9}$
$M_{\odot}$, and concentration, $c_{\rm NFW}\sim13$. Although the agreement
between the model and observations is good, Cloud-9 appears to be slightly, but
systematically, more extended than expected for $\Lambda$CDM RELHICs. This may
imply either that Cloud-9 is much closer than implied by its recessional
velocity, $v_{\rm CL9}\sim300$ 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
$r\lesssim1$ 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 $\Lambda$CDM model on the smallest scales.
