Navarro-Frenk-White (NFW) profile (1996): ρ(r) = ρ_s / [(r/r_s)(1 + r/r_s)²]. Dark matter halos form a universal density profile across 14 orders of magnitude in mass — from dwarf galaxies to galaxy clusters. The concentration parameter c = r₂₀₀/r_s encodes formation history. The left panel shows the halo density in false color; the right shows the rotation curve — the flat tail that first revealed dark matter's existence (Rubin 1970s).