Cosmological backreaction : from the local Hubble expansion rate to dark energy Despite the glorious successes of modern cosmology our understanding of the cosmic substitution is still limited to a tiny fraction (a few per cents only). Accelerated expansion of the Universe caused by the mysterious dark energy is currently the most severe crisis in cosmology even in physics. In this dissertation we argue that light may be shed on this crisis by means of the cosmological backreaction mechanism in the averaging problem in inhomogeneous and anisotropic space time. Due to the non commutation of temporal evolution and spatial averaging the averaged Einstein tensor as the function of the perturbed metric is not trivially equal to the Einstein tensor of the averaged metric. Consequently inhomogeneities and anisotropies (cosmic structures) influence the evolution of the background Universe. In order to obtain the quantitative information of this mechanism we combine Bucherts non perturbative framework with cosmological perturbation theory calculate the relevant averaged physical observables up to third order in the comoving synchronous gauge (both temporal and spatial dependence) and discuss their gauge dependence. With the help of an integrability condition the leading higher order contributions follow from the lower order calculations. We demonstrate that the leading contributions to all the averaged physical observables under consideration are specified completely on the boundary of the averaged domain. For any finite domain these surface terms are nonzero in general and thus backreaction is for real.