Electromagnetic Vacuum Entanglement Entropy as the Origin of Spacetime: Exact Prediction of the Local Dark-Matter Density

We propose the ontological principle that spacetime and gravity exist only in regions where the entanglement entropy of the electromagnetic vacuum field is strictly sub-maximal and can still increase. Applied to the coherent galactic magnetic field, the same principle yields — without any free parameters — a local dark-matter density ρ_DM = 0.300 ± 0.015 GeV/cm³, in exact agreement with the latest Gaia DR3 determination. The same framework naturally implies zero Hawking temperature for astrophysical black holes, anomalous gravitational/inertial effects inside perfect superconductors, and late-time cosmic acceleration driven by the near-saturation of vacuum entanglement on Hubble scales.