This paper proposes a new conceptual framework called the "Rest Energy Hypothesis." It suggests that mass is not an intrinsic property but rather "confined kinetic energy" trapped in closed spacetime orbits. The hypothesis introduces a stability coefficient (η) to quantify the conversion of energy into mass versus spacetime pressure. This framework aims to offer geometric explanations for the nature of mass, the stability of the proton, and the phenomena of Dark Matter and Dark Energy, interpreting them as manifestations of stable versus unstable energy confinement.