  Cosmologists today are confronted with the perplexing reality that the
universe is currently accelerating in its expansion. Nevertheless, the nature
of the fuel that drives today's cosmic acceleration is an open and tantalizing
mystery. There exists the intriguing possibility that the acceleration is not
the manifestation of yet another mysterious ingredient in the cosmic gas tank
(dark energy), but rather our first real lack of understanding of gravity
itself, and even possibly a signal that there might exist dimensions beyond
that which we can currently observe. The braneworld model of Dvali, Gabadadze
and Porrati (DGP) is a theory where gravity is altered at immense distances by
the excruciatingly slow leakage of gravity off our three-dimensional Universe
and, as a modified-gravity theory, has pioneered this line of investigation. I
review the underlying structure of DGP gravity and those phenomenological
developments relevant to cosmologists interested in a pedagogical treatment of
this intriguing model.
