  We propose an effective anisotropic fluid description for a generic
infrared-modified theory of gravity. In our framework, the additional component
of the acceleration, commonly attributed to dark matter, is explained as a
radial pressure generated by the reaction of the dark energy fluid to the
presence of baryonic matter. Using quite general assumptions, and a microscopic
description of the fluid in terms of a Bose-Einstein condensate of gravitons,
we find the static, spherically symmetric solution for the metric in terms of
the Misner-Sharp mass function and the fluid pressure. At galactic scales, we
correctly reproduce the leading MOND-like $\log(r)$ and subleading
$(1/r)\,\log(r)$ terms in the weak-field expansion of the potential. Our
description also predicts a tiny (of order $10^{-6}$ for a typical spiral
galaxy) Machian modification of the Newtonian potential at galactic scales,
which is controlled by the cosmological acceleration.
