This paper formally proposes the Spatial Pressure Medium Gravitational Theory (SGT), addressing the century-old ultimate question in physics: Why does gravity exist? Based on the uniform vacuum background medium—Spatial Pressure Medium (SPM)—the theory establishes a unified, self-consistent, and falsifiable dynamical framework. For the first time, it fully reveals the deep mechanisms of gravitational origin, long-range action, gravitational waves, inertia, and the equivalence principle from the perspective of continuum dynamics, achieving a historic leap from mathematical description to fundamental tracing of gravity. The theory is fully compatible with all observational results of Newtonian gravity and general relativity, abandoning action-at-a-distance and pure geometric abstraction. It provides unique testable predictions such as pressure saturation and mechanical freezing surface in strong-field regimes, opening a new paradigm for gravity and cosmology. Its core picture is simple and profound: The universe is filled with uniform pressure. When matter is placed in it, the pressure becomes non-uniform. This non-uniform pressure field is gravity. Uniform spatial pressure means no gravity; gradient spatial pressure IS gravity!
