  The predictions of homogeneous and isotropic cosmological models with
ordinary matter and gravity are off by a factor of two in the late universe.
One possible explanation is the known breakdown of homogeneity and isotropy due
to the formation of non-linear structures. We review how inhomogeneities affect
the average expansion rate and can lead to acceleration, and consider a
semi-realistic model where the observed timescale of ten billion years emerges
from structure formation. We also discuss the relation between the average
expansion rate and observed quantities.
