  Recent observations provide strong evidence that the universe is
accelerating. This confronts theory with a severe challenge. Explanations of
the acceleration within the framework of general relativity are plagued by
difficulties. General relativistic models require a "dark energy" field with
effectively negative pressure. An alternative to dark energy is that gravity
itself may behave differently from general relativity on the largest scales, in
such a way as to produce acceleration. The alternative approach of modified
gravity also faces severe difficulties, but does provide a new angle on the
problem. This review considers an example of modified gravity, provided by
brane-world models that self-accelerate at late times.
