Conceptual elements of a galaxy: the stars (yellow/blue) and atomic gas (green) of NGC 6946 (Spitzer 3.6μ and 21 cm data: F. Walter et al. 2008) are shown embedded in an extended dark matter halo (black). The dark matter density decreases continuously with radius so the halo has no hard edge, but for convenience, we adopt the common convention that the radius r₂₀₀ marks the boundary of the dark matter halo and the dividing line between the CGM and IGM (orange). The stars and gas illustrated here appear within r < 20 kpc while r₂₀₀ ≈ 220 kpc (not shown to scale).