In fact, current cosmological observations do not agree on the distribution of matter in the present-day universe. The disagreement is quantified using a measure called S8 that characterizes matter's "clumpiness" on a characteristic scale of roughly 8 Megaparsecs (about 26 million light years).

Cosmic microwave background experiments, which collect photons from the early universe and hence are more sensitive to early-time physics, report a higher S8 value than galaxy weak gravitational lensing surveys, which measure galaxy shapes and positions in an older universe and thus are more sensitive to late-time physics. This discrepancy—known as the S8 tension—is part of the broader cosmological crisis, including the H0 tension, where parameters in the standard cosmological model like S8 and H0 do not align between early and late universe observations.

What happens to the late universe?

If of physical origin, what are the implications of the S8 tension to the growth of large-scale structure? A good way to seek a clear answer to this question is to cleanly separate the growth history of cosmic structures, i.e., the clumpy universe, from the expansion history of cosmic background, i.e., the smooth universe.

Within the standard model, almost 70% of the universe is filled with a constant dark energy and this component drives the expansion of the cosmic background. Dark energy and the cosmic background expansion, acting like molasses, naturally suppress cosmic structure growth. That is, they act like a friction against the attraction and clustering of matter. This fact is already accounted for in all cosmological analyses. So the S8 tension indicates a further suppression, beyond the standard model's expectation.