The centers of galaxy clusters are too gentle for cold dark matter. Pure CDM collapse builds cuspy halos whose density rises steeply inward, and at cluster scales the standard expectation steepens further: the brightest cluster galaxy's baryons should adiabatically contract the inner halo, stacking the cusp. The measurements, combining strong lensing arcs, BCG stellar kinematics, and weak lensing into total mass profiles across radial decades, found the opposite: the landmark Newman et al. (2013) analysis of seven massive relaxed clusters showed inner dark-matter profiles significantly shallower than NFW, with mean inner slopes near 0.5 against the predicted unity or steeper, the flattening extending over tens of kiloparsecs around the BCG. Subsequent strong-lensing core measurements and BCG dynamical studies have repeatedly confirmed the genre: massive cluster cores are cored or near-cored exactly where contraction should have made them cuspiest.
The standard repairs invert each other: AGN feedback and dynamical heating by infalling clumps can soften cusps in simulations, but the energetic bookkeeping that cores a 10^15 solar mass halo over tens of kiloparsecs requires feedback far beyond what cores dwarf galaxies, and the same simulations must simultaneously keep the gas quiescent (the measured calm cores) while violently stirring the dark matter. Self-interacting dark matter was partly engineered for this scale, but the cross-sections needed here collide with the merging-cluster offset bounds. The cluster core-cusp problem thus closes the small-scale circle: the same density-profile failure documented in dwarfs reappears at four orders of magnitude greater mass, where the dwarf-scale rescues do not stretch.
The standing is a mature, repeatedly confirmed profile discrepancy at the mass scale least forgiving of baryonic excuses, with JWST-era strong lensing and IFU BCG kinematics steadily tightening the inner slopes.