  Constantly accumulating observational data continue to confirm that about 70%
of the energy density today consists of dark energy responsible for the
accelerated expansion of the Universe. We present recent observational bounds
on dark energy constrained by the type Ia supernovae, cosmic microwave
background, and baryon acoustic oscillations. We review a number of theoretical
approaches that have been adopted so far to explain the origin of dark energy.
This includes the cosmological constant, modified matter models (such as
quintessence, k-essence, coupled dark energy, unified models of dark energy and
dark matter), modified gravity models (such as f(R) gravity, scalar-tensor
theories, braneworlds), and inhomogeneous models. We also discuss observational
and experimental constraints on those models and clarify which models are
favored or ruled out in current observations.
