Failure of the Standard Cosmological Model Beyond Its Epistemic Domain and the Dahli Structural Brane–String Alternative (DCM)

This paper argues that the standard cosmological model (SCM), understood as General Relativity plus ΛCDM plus Quantum Field Theory (GR + ΛCDM + QFT), has a legitimate epistemic domain restricted to Einstein spacetime inside galaxies and to local quantum fields in laboratories. Extending the same metric–field machinery to the vast cosmic vacuum and horizon-scale structure is a strong, unproven extrapolation. We formalise this as a category error: mathematics that is empirically validated for dense, field-dominated regions (P1) is assumed, without independent proof, to also describe the inter-galactic vacuum (P2) and the horizon-scale structure (P3). This leads to what we call a mathematical colonisation of the cosmic vacuum by GR/QFT tools (metric, curvature, quantum fields, entanglement) that may be structurally inappropriate. As an alternative, we introduce a Structural Brane–String Framework, the Dahli Cosmological Model (DCM/DSM), which rebuilds cosmology from mechanical cores (Planck tension, gravitational constant, speed of light) and from the dynamics of membranes and strings, rather than from GR/QFT fields on a smooth manifold. In this framework: • The cosmic vacuum and cosmic energy are structural (absolute) elements: a network of open membranes and open strings. • Dark matter and dark energy are fragile (derived) elements: collapsed membranes and compactified strings. We derive a structural depletion coefficient δ ≡ -ln(1-α ), where α is the fixed point of a non-linear brane phase equation. This δ controls: • Depletion of dark matter (DM) into the cosmic vacuum in the background expansion, • Suppression of structure growth fσ8 (the S8 tension), • A mild deviation of the dark-energy equation of state w(z) from -1. A value δ ≈ 0.3 is naturally obtained from the brane dynamics and is consistent with DESI BAO fits and with an observed ~10% suppression in fσ8. Thus a single structural parameter explains multiple large-scale tensions without introducing extra scalar fields or ad hoc