Segue 1 is barely a galaxy: a few hundred solar luminosities of ancient stars scattered across a system so faint it was initially mistaken for a star cluster. Yet its measured line-of-sight velocity dispersion of 3 to 4.5 km/s, if read as an equilibrium dark-matter-dominated system, implies a mass-to-light ratio of order a thousand, making this nearly invisible object formally the darkest known galaxy (Simon and Geha 2007; Geha et al. 2009).
The inference is as fragile as the system. With only a few dozen measurable member stars, the dispersion is vulnerable to contamination by Sagittarius stream interlopers along the same sightline, to inflation by unresolved binary star orbital motion, and to the possibility that Segue 1 is not in equilibrium at all but tidally disturbed, in which case the dispersion measures disruption rather than mass (Niederste-Ostholt et al. 2009). Within ΛCDM the stakes are high in both directions: if the extreme M/L is real, the model must explain how its smallest halos hold so much dark matter around so few stars, anchoring claims about the minimum halo mass and making Segue 1 a prime target for dark matter annihilation searches (Bonnivard et al. 2015); if it is an artifact, the most-cited ultra-faint benchmark dissolves.
The standing is unresolved at the level of the data themselves: the smallest galaxies are where the dark matter paradigm makes its most extreme claims on its weakest measurements. Thirty-meter-class spectroscopy will eventually multiply the member counts.