  The discovery that the cosmic expansion is accelerating has been followed by
an intense theoretical and experimental response in physics and astronomy. The
discovery implies that our most basic notions about how gravity work are
violated on cosmological distance scales. One simple fix is the introduction of
a cosmological constant into the field equations for general relativity.
However, the extremely small value of the cosmological constant, relative to
theoretical expectations, has led theorists to explore a wide variety of
alternative explanations that involve the introduction of an exotic
negative-pressure fluid or a modification of general relativity. Here we
briefly review the evidence for cosmic acceleration. We then survey some of the
theoretical attempts to account for it, including the cosmological constant,
quintessence and its variants, mass-varying neutrinos, and modifications of
general relativity, such as scalar-tensor and $f(R)$ theories and braneworld
scenarios. We discuss experimental and observational tests that may allow us to
distinguish between some of the theoretical ideas that have been put forward.
