We propose that dark matter is not an exotic new substance, but a geometric necessity of four-dimensional spacetime. Prior to the electroweak Higgs phase transition, all fundamental particles were massless and propagated at the speedof light c. In this pre-Higgs epoch, the four dimensions of spacetime were fully equivalent— no dimension was geometrically privileged as "time". Upon Higgs condensation, the acquisition of mass froze each particle’s instantaneous propagationaxis as its individual time dimension, while the remaining three orthogonal dimensions became its spatial dimensions. Since exactly four basis dimensions are available, this produces four distinct dimensional sectors: one visible sector (our own) and three geometrically equivalent dark sectors. Particles across sectors share thesame four-dimensional manifold and thus interact gravitationally. Electromagnetic interactions— being confined to a shared three-dimensional spatial hypersurface— are geometrically forbidden between sectors. Dark matter thus requires no newparticles, no new forces, and no modifications to gravity: it is a direct consequence of the combinatorial geometry of (3+1)-dimensional spacetime. The predicted dark to-visible matter ratio of 3 : 1 is consistent with cosmological observations. The structural co-distribution of dark and visible matter emerges naturally from their shared gravitational manifold. Furthermore, the hypothesis provides a geometric explanation for the Hubble tension, predicts gravitational wave events without electromagnetic counterparts, and accounts for the persistent null results of all direct dark matter detection experiments.