We present the substrate theory of everything: a unified framework derived from eight ontological postulates that addresses major outstanding problems across quantum mechanics, gravitation, particle physics, and cosmology. The framework rests on a single underlying scalar substrate field with Mexican-hat potential, a U(1) gauge structure producing a cascade of particle masses, and a two-frame ontology distinguishing the undisturbed substrate (F1) from its perturbed observable form (F2). From these postulates we derive: Newton's second law and the equivalence principle from substrate dynamics; special relativity from Doppler shifts of time quanta; the Born rule from F1/F2 sampling combined with envariance and Gleason's theorem; the modified-inertia threshold a₀ = cH₀/(2π) ≈ 1.04 × 10⁻¹⁰ m/s² (matching empirical MOND to 13%); flat galaxy rotation curves and the Tully-Fisher relation; the matter-antimatter asymmetry η_B ≈ 7 × 10⁻¹⁰ from substrate phase-transition dynamics (matching observation to 15%); inflation observables n_s = 0.967, r ≈ 0.003, f_NL ≈ 0.014 from the substrate field with non-minimal coupling (validated against Planck 2018); a candidate solution to the cosmological constant problem reducing the QFT discrepancy from 122 to ~1 orders of magnitude via a holographic bound on F2 substrate displacement (ρ_Λ ~ M_P² H₀²); and a unified treatment of dark energy and dark matter as boundary-scale and cascade-scale manifestations of F2 substrate displacement. The framework explains the double-slit experiment, EPR-Bell correlations, and other quantum-mechanical phenomena through F1's nonlocal but non-signaling structure. The cluster mass gap is filled naturally by uniform cascade dark matter (m_DM ~ 10¹² GeV) without disrupting pure-MOND fits at galactic scales. We list specific predictions for tests through 2045: wide-binary modified-inertia signatures (Gaia DR4, ~2026); B-mode polarization at r ≈ 0.003 (CMB-S4/LiteBIRD, ~2030); 3.5 keV X-ray line from sub-dominant sterile neutrino component (XRISM/ATHENA, 2025-2031); LISA stochastic GW signal at ~1 mHz from cascade phase transition (~2038); dark energy w(z) → -1 with possible few- percent deviations (DESI/Euclid, 2027-2032). The framework spans more than 50 orders of magnitude in scale from Planck physics to the cosmological horizon, while preserving falsifiability at every level. Keywords: theory of everything; substrate framework; modified Newtonian dynamics; cosmological constant problem; dark matter; dark energy; quantum gravity; Born rule; baryogenesis; cascade dark matter; holographic dark energy; F1/F2 ontology