Dark Matter Rotation Curve Simulator (2D)

Orbit Selection

Radius from center 13.0 kpc

Dark Matter Halo

Halo mass fraction 0%

Animation

Orbit speed scale 1.0×

Halo mass fraction 0% — pure Keplerian falloff from visible matter only.

Live Readout

Visible-only speed

— km/s

Speed at this halo %

— km/s

Observed (real) speed

— km/s

Dark matter mass within r

— M☉

How it works

If a spiral galaxy's gravity came only from the stars and gas we can see, orbital speed should fall off with distance the same way planets do around the Sun — a Keplerian decline, v(r) = √(GM_visible(r)/r). Real spiral galaxies, first measured precisely by Vera Rubin in the 1970s, instead keep spinning at almost the same speed all the way to their outer edge.

The halo mass fraction slider blends in an invisible spherical mass component on top of the visible disk: at 0% only the visible mass sets the orbital speed and the curve declines as radius grows; at 100% enough extra mass is added at every radius to exactly reproduce the flat curve astronomers observe. The gap between the two curves at any given radius is the dark matter mass that must exist within that radius for gravity to hold the observed orbit together.

This is the same evidence used across real rotation-curve studies: visible starlight and gas account for only a fraction of the gravitational pull needed to keep outer stars from flying off, so an unseen halo — dark matter — is inferred to make up the difference.

Visible matter only (Keplerian) Observed (real, flat) Current halo fraction