Spacetime Curvature
The heart of general relativity fits in one sentence: mass–energy tells spacetime how to curve, and curved spacetime tells matter how to move. The Earth doesn't pull the Moon with an invisible rope; it dents the spacetime around it, and the Moon coasts along the straightest path through that dent.[1] The familiar "heavy ball on a rubber sheet" picture captures the idea, though it's only an analogy — the real curvature is four-dimensional and includes time, which is what makes things fall rather than float. Where curvature becomes extreme, you get a black hole.
Mass dimples the grid of spacetime into a well — the more mass, the deeper. The two orbiting particles aren't being pulled by a force; they're just coasting through curved geometry — the inner one races while the outer ambles, exactly as inner planets outpace outer ones. Drag the mass to move the well, or slide to change how much it warps: a deeper well quickens both orbits.