The intracluster medium remembers every supernova that ever enriched it: iron and the other heavy elements mixed into the hot gas record the integrated history of stellar nucleosynthesis in and around the cluster. Read carefully, the record tells an impossible story. Metallicity peaks near cluster centers and declines outward, but the gradients run shallower and more uniform than supernova injection from cluster galaxies plus subsequent mixing produces (Leccardi and Molendi 2008; Mernier et al. 2017), and the outer regions are the deeper puzzle: some clusters hold remarkably uniform metallicity beyond 500 kpc, flat profiles at around a third solar extending toward the virial radius.
Uniformity at those radii demands that enrichment happened early and everywhere: metals injected after cluster assembly from localized stellar populations would be clumpy and centrally weighted, so the flat outer profiles require the gas to have been enriched before it fell in, pre-processed to a common metallicity across regions that had never yet shared a structure. ΛCDM owns no mechanism for that: its early universe has too few stars too late to enrich so much gas so evenly, and hierarchical assembly predicts precisely the clumpy, inhomogeneous injection the observations rule out at large radii.
The standing strengthened as XMM and Suzaku pushed metallicity profiles outward, with early-and-everywhere enrichment now the consensus reading of data the consensus model cannot generate.