underdense

Thus the slightly larger expansion rate of the underdense regions makes the measured expansion rate appear to accelerate.

Dark Matter: Still Existing Sean 2007

The idea here is that as structure forms, your overdense regions become smaller, while your underdense regions become larger.

Dark Matter: Still Existing Sean 2007

With a cosmological constant, for instance, the potential becomes shallower as the light traverses it, so the blueshifting as it goes in outpaces the redshifting as it comes out, making for a small blueshifting of the light as it passes through overdense regions, and a small redshifting as it passes through underdense regions.

A Dark, Misleading Force Sean 2007