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.
