If a spiral galaxy contained only the matter we can see — stars, gas, and dust — its rotation curve (orbital speed vs. distance from the centre) should rise near the bulge and then fall off, just like planets slow down the farther they orbit the Sun. When astronomers actually measured real galaxies in the 1970s (most famously Vera Rubin and Kent Ford), they found the curves stayed almost flat out to the visible edge and beyond. This simulation lets you switch a hidden dark matter halo on and off and watch the predicted curve change shape in real time.
v(r) = √(G·M(r)/r) where the enclosed mass M(r) saturates once you're past the stellar disc — so v declines at large radius, the "Keplerian drop-off."M(r) ∝ r — which makes its contribution to v(r) constant, i.e. flat, however far out you go.The mismatch between predicted and observed rotation curves is one of the strongest pieces of evidence for dark matter — an unseen component thought to outweigh ordinary matter in most galaxies by roughly five to one, even though it has never been directly detected.
A spiral galaxy of thousands of orbiting stars, where flipping a dark-matter halo on and off changes how fast the outer stars are able to orbit — visible matter alone predicts a falling curve, but a halo keeps it flat.
Visible-mass-only gravity produces orbital speeds that fall off with radius, just like planets around the Sun. An isothermal dark matter halo adds mass that grows linearly with radius, keeping orbital speed flat — matching real galaxy observations.
Toggle the dark matter halo, adjust its relative mass, change the star count, and speed up or slow down the orbits. Watch the live rotation-curve chart and the disc's outer stars respond in real time.
Vera Rubin and Kent Ford's rotation-curve measurements in the 1970s provided some of the first strong observational evidence that most of a galaxy's mass is invisible.