Quadruply imaged quasars are precision scales for the small-scale structure of lensing galaxies, and the scales keep reading heavy. Smooth mass models that reproduce the positions of a quad lens's four images to milliarcseconds routinely fail to reproduce their brightness ratios: these flux-ratio anomalies, robust at radio and mid-infrared wavelengths where microlensing and dust are negligible, demand additional small-scale structure perturbing the magnifications. In ΛCDM the perturbers are dark subhalos, making anomaly statistics a celebrated dark matter test. The trouble began when the accounting was done: the classic CLASS radio-lens analyses found anomaly frequencies requiring substructure mass fractions several times higher than CDM simulations supply at the relevant radii, and while line-of-sight halos relieve part of the demand, JWST mid-infrared and narrow-line samples continue to find anomalies at rates that press the simulated subhalo abundance.
The bookkeeping is delicate in both directions: baryonic complexity in the lens (disks, edge-on components) explains some anomalies without dark structure, while the dark-substructure reading must reconcile with the same simulations' subhalos being depleted by tidal destruction near exactly the projected radii where quads form. The tension thus mirrors its cluster-scale sibling (the GGSL excess): observed small-scale lensing perturbations, in galaxies as in clusters, run stronger and more frequent than the simulated CDM substructure that was supposed to cause them.
The standing is a quantitative open ledger: JWST is industrializing flux-ratio measurement with mid-infrared precision, time-domain and narrow-line methods are expanding the clean samples, and the substructure demand keeps landing at or above the ceiling of CDM expectations rather than comfortably below it.