  Modified gravity can be considered as an alternative to dark energy. In a
generalized theory of gravity, the universe may accelerate while containing
only baryonic and dark matter. We study, in particular, the evolution of matter
fluctuations in f(R) models within the Palatini approach, and find that the
resulting matter power spectrum is sensitive to a nonlinear dependence on the
curvature scalar in the gravitational action. The constraints that arise from
comparison to the form of the observed matter power spectrum tighten the
previous constraints derived from background expansion by several orders of
magnitude. Models in the allowed parameter space are practically
indistinguishable from general relativity with a cosmological constant when the
backround expansion is considered.
