  There should be two ways to describe the flat rotation curves of galaxies and
cluster of galaxies. Either one can introduce a dark matter component for the
matter filling the halo, or by modifying the gravity theory and give the flat
rotation curve a geometrical nature. Here we adopt an f(R) model suitable for
describing the effect. After matching the solution with the exterior solution,
the effective density, radial and tangential pressures are obtained. Then the
energy conditions and lensing effect is investigated.
