This paper proposes that gravity arises from the constructive interference of de Broglie matter waves in a density-dependent refractive medium. The argument proceeds in five steps: (1) greater mass implies greater density; (2) by Snell's law, greater density implies a higher refractive index; (3) a higher refractive index implies a shorter wavelength; (4) when matter waves propagate outward from an object in all directions, the wavelength is shorter on the side facing the larger mass; (5) constructive interference is stronger and more densely packed where the wavelength is shorter, so the object is preferentially redistributed toward the side of greater mass. This shows gravity to be a wave-mechanical phenomenon: matter is not pulled toward mass by some force; it is redistributed toward mass through the constructive interference of its own matter wave. Newton and Einstein described the trajectory and the geometric structure of gravitational motion; this paper proposes the physical mechanism by which that motion occurs. This paper makes three propositions explicit, with carefully distinguished epistemic status: (A) the mass distribution determines the effective refractive medium through which matter waves propagate — an experimentally established fact, confirmed independently by the COW experiment (1975) and by semiconductor heterojunction experiments confirming Snell's law for matter waves at density boundaries; (B) the displacement of the constructed-interference maximum is the physical mechanism of actual gravitational motion — the paper's sole new physical identification, going beyond Born's rule as standardly stated; it is consistent with but not compelled by the COW data, and constitutes the primary target of experimental falsification (Section 6.6d); (C) the COW fringe shift is a direct measurement of the gravitational mechanism — a logical corollary that follows necessarily from (A) and (B), not an independent assumption. Section 6.6d translates these propositions into five differentiating experimental predictions, ordered by experimental accessibility, including an immediately executable reanalysis of existing COW data and a novel slit-material density experiment that provides a clean differential test between the present framework and standard wave optics.