This thesis investigates aspects of dark matter and dark energy and constraints that can be imposed on them from current and future observations. Specifically, we first study the idea that the observed acceleration of the Universe could be due to the gravitational backreaction of perturbations on superhorizon scales. We show that this does not work for the case of a cosmological model containing baryonic matter, cold dark matter and a scalar field. Next, assuming the presence of dark energy and dark matter, we study the gravitational lensing effects of large scale structures on luminosity distances of sources. Standard candle sources such as supernovae have been used to measure the dark energy content of the Universe, and gravitational lensing is a source of systematic error in these measurements. We investigate the effects of large scale structures like voids and smaller halos using Monte Carlo simulations.