The Rheoviscion: Fundamental Substrate Element with Fixed Physical Properties

This paper introduces the Rheoviscion (Rv), a fundamental physical element of the spacetime substrate. Using measured Hubble constant H₀ = 73.0 km/s/Mpc and critical density ρc = 3H₀²/(8πG), we derive rest mass mRv = 1.1×10⁻⁴⁹ kg, number density n = 9.09×10²² m⁻³, and spacing d = 22.2 nm with zero adjustable parameters. The substrate exhibits non-Newtonian rheology with slip parameter Γ = 0.00035, critical Reynolds number Recrit = 2857, and critical vorticity χc = 1.12×10⁷⁹ s⁻¹. The Rheoviscion is required by seven independent observations: solar wind momentum excess (AAS74463), Io plasma torus 0.6 TW deficit (AAS74050), Milky Way flat rotation without dark matter (AAS74360), Hubble tension resolution (AAS74318), 3I/ATLAS non-gravitational acceleration (AAS74269), CMB polarization rotation 0.28° (AAS74214), and ultra-high-energy gamma-ray spectral index (AAS74141). These span 44 orders of magnitude (10⁻¹⁸ m to 10²⁶ m) across 28 domains documented in SVCF Unified Field Theory (Zenodo 10.5281/zenodo.18848748). Three falsification tests: RHIC Λ-Λ̄ correlations (C = -1.000), 3I/ATLAS perijove 52,400 km versus standard 53,587 km (testable March 16, 2026), and THz resonator collapse at 2857 cycles. The Rheoviscion is not a quasiparticle, not emergent, and not dark matter. If it exists, dark matter is unnecessary. If it does not, seven independent observations remain unexplained.