The physical origin of dark energy remains unknown. The standard ΛCDM model treats dark energy as a cosmological constant, ρ_Λ ≈ 5.9×10^{-10} J/m^3, but cannot explain why this value occurs. We propose the Cashmere Ball Law: the local dark energy density is determined by the total effective blocking area of the Solar System's Oort cloud. The name reflects the fluffy, fibrous structure of the Oort cloud — like a cashmere ball, where countless tiny fibers (cometary nuclei) collectively block the KuiQuark flow. Combining astronomical observations of the Oort cloud (N ~ 10^{12} cometary nuclei, r ~ 1 km) with first-principles constants (E_P, c, v_⊙, ε = 1/137), we derive ρ_Λ = (E_P / A_eff) (v_⊙/c)^2 ε^{-4}, where A_eff = η A_geo is the total effective blocking area, A_geo = N π r^2 ≈ 3.14×10^{18} m^2, and η ≈ 200 is the inferred blocking efficiency. The Cashmere Ball Law naturally explains the observed dark energy density without free parameters. It passes an independent cross-scale test: for the Local Group of galaxies, Chernin et al. (2009) measured ρ_Λ^{Local} = (0.9±0.3) ρ_Λ^{global}, in agreement with the law's prediction. This paper is part of the XLG-CM series (XLG Cosmology — Celestial Motion). Part of XLG Cosmology — Eccentric Collision Universe (ECC).