Density-Triggered Metastable Vacuum Energy from an Avoided Bosonic Spectrum

Why matter and vacuum energy become comparable only at late times remains unexplained. We present dark-state capture (DSC), a matched dark-sector effective field theory in which cosmic dilution of a cold coherent carrier stabilizes a metastable vacuum. A compact scalar controls an avoided spectrum of two bosonic states. The parity-stable lower eigenstate behaves as cold matter and contributes its rest energy to the scalar effective potential. As its adiabatically conserved number density redshifts, the symmetric point changes from a maximum to a metastable minimum. In the benchmark, the transition occurs at a carrier-to-false-vacuum density ratio of 0.990099; this near equality is a non-universal consequence of order-one matching. The same occupied state therefore supplies both the matter density and the density-dependent scalar curvature, while exact exchange/reflection symmetry forces its linear scalar charge to vanish at the captured limit. We derive the canonical action, covariant exchange and perturbation equations, and the adiabatic single-state reduction. The benchmark has positive kinetic terms, nonnegative squared propagation speeds after capture, negligible eigenstate leakage, and finite homogeneous and one-dimensional nonlinear capture domains. At a calibrated reference epoch, the high-wavenumber fifth-force fraction is 0.462%, and a relativistic calculation gives sub-percent differences from the exactly symmetric internal reference over six sampled modes. These are consistency diagnostics, not observational residuals. DSC converts the timing coincidence into an order-one matching relation, but it neither predicts the absolute vacuum scale nor supplies a radiatively natural ultraviolet completion.