Expanding—Contracted Space Theory (ECST) reframes gravity, cosmology, and particle physics by treating space as a cellular medium with variable density rather than an empty backdrop. A scalar contraction field governs cell volume: its gradients reproduce gravity, electromagnetic fields contract space directionally, matter induces spherical contraction on average, and space cells in relaxed, low-density regions (cosmic voids) replicate at a universal rate. From these mechanisms emerge a unified explanation of major puzzles across scales:u2022Cosmology. Photon redshift and late-time acceleration arise from space replication in cosmic voids, resolving the Hubble tension without dark energy while preserving CMB and distance-duality tests.u2022Astrophysics. In dense regions, screening restores General Relativity. In galaxies, unscreened gradients produce MOND-like accelerations and the baryonic Tully—Fisher relation without dark matter. In black holes, collapse halts at a finite density ceiling, eliminating singularities and preserving external Kerr/Schwarzschild metrics.u2022Microphysics. The same sextic elastic potential that caps black-hole collapse yields stable soliton excitations corresponding to the electron, muon, and tau, naturally generating the charged-lepton family. Neutral solitons stabilized by a quartic gradient generate neutrino masses with no per-flavor tuning. When collective SU(2) and SU(3) rotations of the same solitonic core are quantized, the baryon octet and decuplet emerge with GMO and equal-spacing mass relations. Canonical electromagnetic normalization fixes the dielectric response of the medium, eliminating ad-hoc parameters and ensuring that charge and mass hierarchies are derived rather than imposed.With only a small set of universal constants, ECST links particle spectra, hadron structure, galactic dynamics, cosmic redshift, and compact-object physics under one density-based action principle. It reproduces all local precision tests, predicts measurable deviations in gravitational-wave ringdowns and void-sensitive redshift surveys, and offers a parameter-economical, falsifiable alternative to ΛCDM, dark matter, and arbitrary mass hierarchies in the Standard Model.