The Ursa Minor dwarf spheroidal carries fossils that should have dissolved. Photometric and kinematic mapping reveals a pronounced secondary density peak offset from the galaxy's center, along with other clumpy stellar substructures that appear kinematically cold, small internal velocity dispersions in groups of stars moving together, and old, implying survival across many crossing times (Kleyna et al. 2003; Pace et al. 2014).
In ΛCDM the persistence is the problem. A dwarf spheroidal this dark-matter-dominated should sit in a cuspy halo whose tidal field shears cold clumps apart within a few orbital periods, while the predicted population of orbiting subhalos stirs and heats whatever shear misses. N-body work quantifies the verdict: keeping Ursa Minor's secondary peak intact for a Hubble time requires a large, nearly harmonic constant-density core, a potential in which clumps orbit without shearing, and excludes the cuspy profile standard simulations produce (Lora et al. 2012). The clump thus joins the core-cusp and substructure tensions from an independent direction: it is not a profile fit arguing for a core but a surviving structure testifying to one (Bullock and Boylan-Kolchin 2017).
The standing is a clean dynamical witness: something fragile has lived a long time in a potential the standard model says should have destroyed it. Deeper photometry and Gaia-era kinematics keep sharpening both the clump's reality and its coldness.