The S8 tension is the 3–4s discrepancy between the relatively low amplitude of matter clustering inferred from late-time probes such as weak gravitational lensing and galaxy clustering (e.g., KiDS, DES, HSC, DESI), and the higher S8 value predicted when Planck CMB data are interpreted within the standard ΛCDM model (Di Valentino 2021; Asgari et al. 2021). Late-universe measurements consistently indicate suppressed structure growth on ~8 Mpc scales compared to the ΛCDM expectation calibrated by the early universe, yet the overall expansion history and CMB lensing remain close to Planck ΛCDM, implying that the model’s simple link between early-time initial conditions and late-time clustering may be incomplete (Amon & Efstathiou 2022; Poulin et al. 2023).