The brightest satellites the Milky Way should have are missing, and the ones it has are too light for their predicted homes. Boylan-Kolchin, Bullock and Kaplinghat (2011, 2012) identified the problem in the Aquarius and Via Lactea simulations: the roughly ten most massive subhalos of a Milky-Way-mass host are so dense that dwarf galaxies forming within them would have central velocity dispersions and rotation speeds well above anything observed among the actual bright satellites. These subhalos are too big to fail: too massive for reionization or feedback to have kept dark, yet no observed satellite matches their predicted kinematics. The discrepancy reappears in Andromeda's satellites and among field dwarfs of the Local Group, where environmental stripping defenses cannot reach.
The standard responses each carry a cost. Baryonic feedback that cores out the dense subhalo centers requires star formation efficient enough to reshape the halo in systems whose entire stellar mass is a million suns, an energetic stretch at the faint end; a lighter Milky Way halo relieves the count but conflicts with stream and escape-velocity masses; and stripping arguments fail for the isolated field dwarfs showing the same kinematic shortfall. The persistence of the problem across environments suggests the dense predicted subhalos simply do not exist as built: the central densities of small ΛCDM halos are systematically too high, connecting too-big-to-fail to the core-cusp problem as one density crisis.
The standing is a canonical small-scale tension, two decades old, alive in every review: its environmental universality is the sharpest constraint, and Rubin-LSST kinematic censuses of Local Group and field dwarfs will fix the dense-subhalo deficit with population statistics.