If Lorentz transformation equations is vacuum energy dependent which lead to the wave-particle duality and Heisenberg uncertainty principle, and then photon mediates gravitation according to this transformation and the new equivalence principle in which acceleration equivalent to vacuum fluctuations. The uncertainty principle requires every physical system to have a zero-point energy greater than the minimum of its classical potential well. Thus the zero point energy explains the rotation curve of a typical spiral galaxy: the 'flat' appearance of the velocity curve out to a large radius.
http://dx.doi.org/10.14299/ijser.2014.10.002