🌌 Dark Matter — Galaxy Rotation Curves
One of the strongest pieces of evidence for dark matter comes from galaxy rotation curves. Stars at the outskirts of galaxies orbit much faster than Kepler's laws predict from visible mass alone. Explore the NFW dark matter halo profile and see how it fills the gap.
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Baryonic Mass
Dark Matter Halo (NFW)
Legend
ρ(r) = ρ₀ / [(r/r_s)·(1+r/r_s)²]
v_halo² = GM_halo(r)/r
v_total² = v_disk²+v_bulge²+v_halo²
Vera Rubin (1970s) confirmed
flat curves in 200+ galaxies.
The Dark Matter Problem
According to Newtonian gravity and the visible matter in a galaxy (stars, gas, dust), orbital speeds should fall off as v ∝ r-½ at large radii — just like planets around the Sun. But observations by Vera Rubin and Kent Ford in the 1970s showed that rotation curves of spiral galaxies are flat out to many scale radii. The only way to explain this is to postulate a large, extended mass distribution — a dark matter halo — that adds mass even where no stars are visible. The NFW (Navarro-Frenk-White) profile describes the density distribution predicted by cold dark matter simulations: ρ(r) = ρ₀ / [(r/r_s)(1+r/r_s)²]. About 85% of all matter in the Universe is thought to be dark matter, yet its nature remains unknown.