Dark Energy: a Brief Review

The problem of dark energy is briefly reviewed in both theoretical and observational aspects. In the theoretical aspect, dark energy scenarios are classified into symmetry, anthropic principle, tuning mechanism, modified gravity, quantum cosmology, holographic principle, back-reaction and phenomenological types. In the observational aspect, we introduce cosmic probes, dark energy related projects, observational constraints on theoretical models and model independent reconstructions.Comment: 19 pages, invited review article to appear in the special issue of "Frontiers of Physics" dedicated to "High energy astrophysics", an shortened version of our previous article arXiv:1103.587