Leo T is the faintest gas-rich galaxy known: an ultra-faint Local Group dwarf at roughly 420 kpc, near the Milky Way's virial radius, holding onto a reservoir of cold neutral hydrogen and quietly forming stars despite a stellar mass of only a few hundred thousand suns (Irwin et al. 2007; Ryan-Weber et al. 2008). Everything about it is marginal, which is what makes it diagnostic.
The standard model finds Leo T over-constrained. Its survival as a gas-bearing system requires a dark halo massive enough to shield the gas through cosmic reionization, which should have evaporated the reservoirs of halos this small; its ongoing star formation requires gas dense enough to collapse despite global stability criteria suggesting it should not; its gentle orbit must have avoided the halo passages that would strip it; and its HI and stellar components show subtle spatial and kinematic offsets that a relaxed equilibrium system should not maintain (Simon 2019; Read et al. 2024). Each requirement individually fits inside ΛCDM's flexibility; jointly they demand a halo structure, feedback history, and environment tuned to thread the eye of several needles at once.
The standing makes Leo T a benchmark for the smallest scales of galaxy formation: the system where reionization suppression, dark halo structure, and gas physics intersect most tightly. Deeper HI mapping and JWST color-magnitude depths keep refining each constraint.