  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.
