  The collision-free Boltzmann equation is used in the context of brane-$f(R)$
gravity to derive the virial theorem. It is shown that the virial mass is
proportional to certain geometrical terms appearing in the Einstein field
equations and contributes to gravitational energy and that such a geometric
mass can be attributed to the virial mass discrepancy in a cluster of galaxies.
In addition, the galaxy rotation curves are studied by utilizing the concept of
conformal symmetry and notion of conformal Killing symmetry. The field
equations may then be obtained in an exact parametric form in terms of the
parameter representing the conformal factor. This provides the possibility of
studying the behavior of the angular velocity of a test particle moving in a
stable circular orbit. The tangential velocity can be derived as a function of
the conformal factor and integration constants, resulting in a constant value
at large radial distances. Relevant phenomenon such as the deflection of light
passing through a region where the rotation curves are flat and the radar echo
delay are also studied.
