  The nature of the fuel that drives today's cosmic acceleration is an open and
tantalizing mystery. The braneworld theory of Dvali, Gabadadze, and Porrati
(DGP) provides a context where late-time acceleration is driven not by some
energy-momentum component (dark energy), but rather is the manifestation of the
excruciatingly slow leakage of gravity off our four-dimensional world into an
extra dimension. At the same time, DGP gravity alters the gravitational force
law in a specific and dramatic way at cosmologically accessible scales. We
derive the DGP gravitational force law in a cosmological setting for spherical
perturbations at subhorizon scales and compute the growth of large-scale
structure. We find that a residual repulsive force at large distances gives
rise to a suppression of the growth of density and velocity perturbations.
Explaining the cosmic acceleration in this framework leads to a present day
fluctuation power spectrum normalization sigma_8 <= 0.8 at about the two-sigma
level, in contrast with observations. We discuss further theoretical work
necessary to go beyond our approximations to confirm these results.
