The flat rotation curve of the Milky Way is traditionally attributed to a dark matter halo. We propose an alternative physical mechanism: the Slip-Friction Law. The Milky Way is embedded in the KuiQuark Sea (XLG) — a viscous dark fluid with a non-zero background tangential velocity v_bg. Viscous friction transfers angular momentum from the rotating disk to the background medium, forcing the outer rotation velocity to approach v_bg. From the steady-state angular momentum diffusion equation, we derive the rotation curve: v_φ(R) = v_core (R_core/R) + v_bg (1 - R_core/R), where v_core = 220 km/s at R_core = 8.2 kpc. Fitting to the observed rotation curve (Eilers et al. 2019) yields v_bg = 193 ± 12 km/s. The CMB dipole (Planck 2018) gives a heliocentric velocity of ~370 km/s; its projection onto the Galactic plane yields ~200 km/s, consistent with v_bg. The Slip-Friction Law requires one free parameter, while the NFW dark matter halo requires two. Model comparison favors the Slip-Friction model (ΔAIC = AIC_SF − AIC_NFW = −2.3); while not overwhelming, the difference favors the one-parameter model. These results suggest that galactic rotation curves may arise from viscous coupling to a cosmic medium rather than from dark matter. This paper is part of the XLG-CM series (XLG Cosmology — Celestial Motion). Part of XLG Cosmology — Eccentric Collision Universe (ECC).