In ΛCDM, the late Integrated Sachs–Wolfe (ISW) effect should produce a measurable positive cross-correlation between CMB temperature maps and tracers of large-scale structure, yet many galaxy and void-based measurements find amplitudes that are lower than, or only marginally consistent with, the standard ΛCDM expectation (Crittenden & Turok 1996; Fang et al. 2019). This persistent tendency toward a weaker ISW–galaxy signal, and the scatter between different tracers and analyses, complicates the interpretation of dark energy and growth of structure, forcing ΛCDM either to appeal to subtle survey systematics or to consider nontrivial extensions such as evolving dark energy, modified gravity, or nonstandard large-scale potentials (Granett et al. 2008; Manzotti & Dodelson 2014).