  Recently, we have derived a novel and compact expression for how
perturbations in the matter fields of the cosmological fluid can lead to
deviations from the standard Friedmann equations. Remarkably, the dissipative
damping of velocity perturbations by bulk and shear viscosity in the dark
sector can modify the expansion history of the universe on arbitrarily large
scales. In universes in which this effect is sufficiently sizeable, it could
account for the acceleration of the cosmological expansion. But even if dark
matter should be less viscous and if the effect would be correspondingly
smaller, it may have observable consequences in the era of precision cosmology.
Here, we review the origin of this backreaction effect and possibilities to
constrain it further.
