The object of this article is to present a new theory based on the introduction of a non-material medium which makes it possible to obtain a Preferred Frame of Reference (in the context of special relativity) or a Reference (in the context of general relativity), that is to say a dynamic medium of reference. The theory of the dynamic medium of reference is an extension of Lorentz–Poincaré’s theory in the domain of gravitation in which instruments (clocks, rulers) are perturbed by gravitation and where only the measure of the speed of light always gives the same result. The presence of a massive body creates a centripetal flux of the medium, which has three fundamental effects: the dilatation of the period of material clocks, the contraction of the length of material rulers, and the... slowdown of light. Thanks to the centripetal flux of the medium and these three effects, it is possible to find the correct expression of the deflection of a ray of light and the Shapiro delay. The dynamic medium of reference allows to establish a gravitational transformation and to find the fundamental equations of movement for light and matter. Hence, the theory of the dynamic medium of reference allows to find the main results of general relativity, but with important differences: The simultaneity is absolute, existence of the Preferred Frame of Reference, the physical reality is the universal present moment and not a global space-time (block-universe), light is slowed down by a gravitational field.