Void stacking is the trick that makes the faint visible: no single void imprints a measurable temperature signal on the CMB, but align hundreds of survey-identified voids and average the microwave sky behind them, and the composite reveals whether underdense regions systematically cool the photons crossing them. The expected mechanism is the integrated Sachs-Wolfe effect, with an amplitude ΛCDM computes directly from the matter underdensity and the standard potential-decay rate.
The stacks come back too cold. From the original supervoid stacking detection onward, composite void profiles show a cold bias exceeding the ΛCDM ISW prediction by factors of several, a result that has survived changes of void catalog, CMB map, and stacking methodology even as its precise amplitude remains debated (Granett et al. 2010; Nadathur and Hotchkiss 2015). Within the standard model the excess is stranded: the ISW amplitude is fixed by the potential-decay rate of a constant-Lambda background, leaving only the appeal that the stacked voids are statistical flukes of selection, an argument that strains as the result recurs across independent catalogs. The same sector simultaneously reports the sky-averaged ISW cross-correlation running low, so the model's one decay rate is failing in both directions at once.
The standing is a quantitative argument about amplitude rather than existence: the cold imprint is there, and the question is whether any standard accounting reaches it. DESI's void catalogs crossed with Simons Observatory and CMB-S4 maps will fix the stacked amplitude decisively.