  The backreaction of structure formation influences the cosmological evolution
equation for the homogenous and isotropic average metric. In a cold dark matter
universe this effect leads only to small corrections unless a substantial
fraction of matter is located in regions where strong gravitational fields
evolve in time. A``cosmic virial theorem'' states that the sum of gravitational
and matter pressure vanishes and therefore relates the average kinetic energy
to a suitable average of the Newtonian potential. In presence of a scalar
``cosmon'' field mediating quintessence, however, cosmology could be modified
if local cosmon fluctuations grow large. We speculate that this may trigger the
accelerated expansion of the universe after the formation of structure.
