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Based on a new H i survey using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), combined with the Pan-STARRS1 images, we identified an isolated H i cloud without any optical counterpart, named FAST J0139+4328. The newly discovered H i cloud appears to be a typical disk galaxy since it has a double-peak shape in the global H i 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 M _⊙ ). Furthermore, we obtained that the H i mass of this galaxy is (8.3 ± 1.7) ×10 ^7 M _⊙ , 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.