  Observations of the expansion rate of the universe at late times disagree by
a factor of 1.5-2 with the prediction of homogeneous and isotropic models based
on ordinary matter and gravity. We discuss how the departure from linearly
perturbed homogeneity and isotropy due to structure formation could explain
this discrepancy. We evaluate the expansion rate in a dust universe which
contains non-linear structures with a statistically homogeneous and isotropic
distribution. The expansion rate is found to increase relative to the exactly
homogeneous and isotropic case by a factor of 1.1-1.3 at some tens of billion
of years. The timescale follows from the cold dark matter transfer function and
the amplitude of primordial perturbations without additional free parameters.
