Abstract Modern physics is characterized by powerful but disparate theories, from General Relativity to String Theory. This paper presents Expansion Theory, a unifying framework that proposes a single, geometric origin for the cosmos. The theory posits that reality is constructed from discrete Dimensional-Decahedrons (D-Decahedrons), which act as the fundamental containers for all physical laws and phenomena. Each D-Decahedron generates both gravity and cosmic expansion via a dual-pole system of attractive forces. Internally, the D-Decahedron houses a tiered system of 12 Frequency Layers, regulated by a dynamic Harmonic Magnetic Field. Crucially, the theory provides a physical context for String Theory: the fundamental strings of matter are encapsulated and suspended in stable resonance pockets created by the interference of the frequency layers. This model redefines the classical singularity as a state of maximum geometric compression, thereby containerizing the principles of cosmology, quantum mechanics, and string theory within a single, cohesive architecture.