ABSTRACT The nature of dark energy can be probed by the derivative at redshift z = 0 of the deceleration parameter q(z). It is probably static if or dynamic if , supporting ΛCDM or , respectively, where H denotes the Hubble parameter. We derive , enabling a determination of q(z) by measuring Milgrom’s parameter, , in galaxy rotation curves, equivalent to the coefficient A in the Tully–Fisher relation between a rotation velocity V c and a baryonic mass M b . We infer that dark matter should be extremely light, with clustering limited to the size of galaxy clusters. The associated transition radius to non-Newtonian gravity can conceivably be probed in a freefall Cavendish-type experiment in space.