  Based on a new HI survey using the Five-hundred-meter Aperture Spherical
radio Telescope (FAST), combined with the Pan-STARRS1 images, we identified an
isolated HI cloud without any optical counterpart, named FAST J0139+4328. The
newly discovered HI cloud appears to be a typical disk galaxy since it has a
double-peak shape in the global HI profile and an S-like rotation structure in
the velocity-position diagram. Moreover, this disk galaxy has an extremely low
absolute magnitude (M_B>-10.0 mag) and stellar mass (<6.9*10^5 Msun).
Furthermore, we obtained that the HI mass of this galaxy is 8.3*10^7 Msun, and
the dynamical mass to total baryonic mass ratio is 47+-27, implying that dark
matter dominates over baryons in FAST J0139+4328. These findings provide
observational evidence that FAST J0139+4328 is an isolated dark dwarf galaxy
with a redshift of z=0.0083. This is the first time that an isolated dark
galaxy has been detected in the nearby universe.
