Galaxy clusters lens their own galaxies more efficiently than simulated clusters know how to. Galaxy-galaxy strong lensing, the production of arcs and multiple images by individual cluster member galaxies rather than by the cluster as a whole, is a clean count: identify the events, compare to the rate state-of-the-art simulations produce. Meneghetti et al. (2020, Science) performed the comparison across eleven Hubble Frontier Fields-quality clusters and found observed GGSL probabilities exceeding ΛCDM expectations by more than an order of magnitude, a discrepancy the authors framed as a systematic issue with simulations or deeper. Follow-up with full baryonic physics (Ragagnin et al. 2022) narrowed the gap to a persistent factor of 2 to 4, and the effective Einstein radii tell the same story in physical terms: observed member-galaxy substructures lens with theta_E of 2 to 5 arcseconds where simulated counterparts manage 0.3 to 1.
The repair inventory is short. Boosting subhalo concentrations helps but no identified CDM mechanism supplies the required compactness at the right masses without violating field-galaxy constraints; resolution and baryonic-implementation arguments have absorbed part but not all of the excess; and SIDM moves the wrong direction, making subhalos puffier. The anomaly is the cluster-scale member of a family: lensed-quasar flux ratios at galaxy scales and the broader substructure-lensing ledger all find small-scale gravitational perturbations stronger than simulated CDM substructure provides, inverting the missing-satellite intuition, where the model has too much substructure by count, it has too little by lensing punch.
The standing is an active quantitative crisis: JWST cluster imaging is multiplying the clean GGSL census, the factor 2-to-4 residual has survived its baryonic audit, and the compactness of cluster galaxy substructure stands as a direct, repeatable challenge to CDM's small-scale predictions.