I propose a novel theoretical framework in which dark matter originates from the hidden gravitational mass associated with ordinary photons. Through a string-scale entanglement with the Higgs field, photons acquire a tiny, scale-dependent effective gravitational mass. The enormous number of photons in the observable universe (approximately 1.47 x 10^89) produces a cumulative gravitational effect that naturally accounts for the observed dark matter density (Ω_DM) without requiring any new elementary particles. To ensure sufficient dark matter during the critical epoch of cosmic structure formation, the model includes a transient early-universe component called the Genesis Field. This component later transitions smoothly to the dominant photon-string-Higgs entanglement mechanism at late times. The model reproduces the observed dark-to-baryonic matter ratio (Ω_DM/Ω_b ≈ 5.5) and remains compatible with key precision observations while offering testable predictions in both cosmology and laboratory experiments. It further predicts the possible reconversion of accumulated dark matter back into ordinary radiation during extreme astrophysical events such as neutron star mergers or black hole collisions. Main updates in Version 1.8 compared to Version 1.8: Changes in Version 1.8 * **Title Update:** Revised the title to "Photon-String-Higgs Coupling and the Genesis Field -- Why Dark Matter Could Consist of Ordinary Light" to precisely reflect the expanded focus on the field-theoretic framework.* **Introduction of a Field-Theoretic Screening Mechanism:** Added a mathematically formalised, density- and radiation-flux-dependent screening mechanism to resolve the tension between cosmological photon mass requirements (~3 x 10^-3 eV) and tight terrestrial/solar system laboratory bounds.* **Formalisation of the Genesis Field & Effective Lagrangian:** Section 2 has been expanded to include the explicit field-theoretic framework, providing the effective Lagrangian density coupling the Genesis Field, the Photon-String-Higgs sector, and baryonic matter.* **Mathematical Definition of the Ilias-Lucida Threshold (\rho_IL):** Introduced the effective potential governing the density-dependent mass suppression. Formulated the critical transition parameter \rho_IL to model the spatially-sigmoid phase transition at the heliopause (r \approx 120 AU).* **Empirical Data Calibration:** Connected the model's theoretical gradients to in-situ plasma wave science (PWS) telemetry from Voyager 1 and Voyager 2, and established the 21-cm absorption feature of the EDGES signal (z \approx 20) as a cosmological anchor.* **Updated Figures and Plots:** Revised all graphics to visualize the sigmoid phase transition at the heliopause and updated the data curves to match the latest Voyager 1/2 PWS measurement data.* **Refined Tables:** Adjusted physical parameter tables, coupling constants, and density gradients to reflect the calibrated Ilias-Lucida threshold values.* **Editorial & Metadata Updates:** Updated publication date, versioning metadata, and corrected ORCID hyperref structures in the LaTeX frontmatter.

Note: This preprint was developed with assistance of the AI Grok (xAI) and Google KI. Full acknowledgments are included in the document