The Milky Way's brightest satellites do not scatter around the Galaxy the way accreted substructure should; they organize. Eleven or more of the classical dwarfs lie in a remarkably thin plane roughly polar to the Galactic disk, the Vast Polar Structure, and proper motions show several sharing similar orbital poles, orbiting within the plane rather than merely passing through it (Pawlowski et al. 2015).
In ΛCDM, satellites populate a roughly triaxial dark matter halo, accreted stochastically with only mild anisotropy from filamentary infall, so thin long-lived co-rotating planes are rare, occurring in well under one percent of simulated Milky Way analogs once kinematic coherence is required (Pawlowski 2018; Bullock and Boylan-Kolchin 2017). The presence of the massive Large Magellanic Cloud complicates the picture further, perturbing any fragile alignment. And the Milky Way is not alone: comparable co-rotating planes around M31, Centaurus A, and other hosts drive the joint probability of the observed ensemble down toward one in ten trillion under standard assumptions (Kroupa et al. 2024). Transient-alignment defenses constructed for the Milky Way do not transfer to the ensemble.
The standing is among the sharpest small-scale challenges to the standard model because it is geometric and kinematic at once: position planes might be projection, but shared orbital poles are dynamics. LSST's satellite census around many more hosts converts the question into a frequency measurement.