Disk Galaxy Rotation — With and Without Dark Matter

Both galaxies start identical: same stars, same spiral pattern, same visible mass. The left galaxy sits inside a dark matter halo, so its rotation curve stays flat and the outskirts keep pace. The right galaxy has only its visible matter; outer stars orbit slower and slower (Keplerian falloff), the outskirts visibly lag, and the spiral pattern shears apart differently. The chart below shows the rotation curves driving each simulation — the flat observed curve is the classic evidence for dark matter.

With dark matter halo flat rotation curve — what we actually observe

Visible matter only Keplerian falloff — what Newton predicts without dark matter

Rotation curves orbital speed vs. distance from galactic center

Total with dark matter (flat)
Visible matter only (falls off)
Dark matter halo contribution

Controls

Stars are colored by orbital speed (blue = slow, white/yellow = fast). Halo strength scales the dark matter halo on the left galaxy and in the chart; drag it to zero and the two galaxies match.