Distance determinations of extremely-low-surface-brightness galaxies are expensive in terms of spectroscopic time .
Because of this , their distances are often inferred by associating such galaxies with larger structures such as groups or clusters , leading to a systematic bias by selecting objects in high density environments .
Here we report the discovery of a red ultra-diffuse galaxy ( S82-DG-1 : r _ { \mathrm { eff } } = 1.6 kpc ; < \mu _ { g } > = 25.7 mag arcsec ^ { -2 } ; g - i = 0.78 mag ) located in a nearby cosmic void .
We used multi-band luminosity functions of its globular clusters to obtain the distance to S82-DG-1 , at 28.7 _ { -3.6 } ^ { +4.2 } Mpc .
Follow-up deep spectroscopy with the GTC telescope yields a redshift of 3353 \pm 29 km s ^ { -1 } , making its association with the NGC 1211 galaxy ( one of the most isolated galaxies known ) highly likely .
Both galaxies have compatible distances and redshifts , share a high peculiar velocity ( \sim 1000 km s ^ { -1 } ) and lie within a void of radius 7 Mpc .
The local surface density is \Sigma _ { 5 } ~ { } \sim~ { } 0.06 Mpc ^ { -2 } , an order of magnitude smaller than the field population and similar to the voids found in the GAMA survey .
Our work shows : i ) The high potential of using optical ground-based photometry of associated globular clusters to explore distances to ultra-diffuse galaxies and ii ) the presence of red ultra-diffuse galaxies even in the most sparse environments , suggesting a wide range of formation mechanisms .