If galaxies contained only the matter we can see, their outer stars should orbit slower the farther out they sit — just as planets orbiting the Sun do. Real spiral galaxies stubbornly refuse to obey this rule: their rotation curves stay roughly flat. This simulation lets you toggle an invisible dark matter halo on and off to see exactly why.
Orbital velocity depends on the mass enclosed within an orbit's radius: v = √(GM(r)/r). For visible matter alone, enclosed mass saturates near the galaxy's edge, so velocity should fall off like a Keplerian 1/√r curve at large radii. Observed velocities instead stay flat, implying a large reservoir of unseen mass — a dark matter halo — whose enclosed mass keeps growing with radius.
Toggle the dark matter halo on and off and watch both the galaxy's outer stars and the live rotation-curve graph respond instantly. Adjust halo mass and extent to reshape the flat curve, and change visible star count and visible mass to see how much luminous matter alone can explain — and how much it cannot.
Astronomer Vera Rubin's precise rotation-curve measurements of spiral galaxies in the 1970s provided the decisive observational evidence for dark matter, which is now estimated to make up about 27% of the universe's total mass-energy — roughly five times more than all visible matter combined.
This galaxy rotation curve simulation pairs a top-down spiral galaxy with a live-updating graph of orbital velocity versus radius, v(r) = √(GM(r)/r). Visible matter is modelled as an exponential disc whose enclosed mass saturates near the edge, producing a Keplerian decline at large radii — the red curve. A toggleable dark matter halo adds an enclosed mass that keeps growing roughly linearly with radius (an isothermal-sphere-like profile), producing the flat blue curve that actually matches real astronomical observations of spiral galaxies.
With the halo switched off, outer stars in the simulated galaxy visibly slow down as they orbit, mirroring the Keplerian decline expected from visible matter alone. Switching the halo on speeds those same outer stars back up to match an observed flat rotation curve, illustrating exactly the mismatch that led astronomers to infer the existence of dark matter.
Halo mass and Halo extent reshape the invisible mass component and the flatness of the blue curve. Visible stars sets how many star points populate the disc, and Visible mass scales the luminous matter that produces the red Keplerian curve. Toggle the Dark Matter Halo button to compare both scenarios directly.
Vera Rubin and Kent Ford's rotation-curve measurements of the Andromeda Galaxy and dozens of other spirals in the 1970s were so precise and consistent that they turned dark matter from a fringe idea, first suggested by Fritz Zwicky in 1933, into mainstream astrophysics within a decade.
A rotation curve plots the orbital velocity of stars and gas in a galaxy against their distance from the galactic centre. For an object orbiting a central mass, physics predicts velocity should decrease at large distances (a Keplerian decline), similar to how outer planets orbit the Sun more slowly than inner ones.
If a galaxy's total mass matched its visible starlight and gas, orbital velocities should fall off at large radii. Instead, precise measurements — most famously by Vera Rubin and Kent Ford in the 1970s — show velocities staying roughly constant far beyond where visible matter thins out, implying a substantial amount of unseen mass extending well past the visible disc.
A dark matter halo is a roughly spherical distribution of invisible, non-luminous mass thought to surround galaxies, extending far beyond their visible stars and gas. It does not emit, absorb or reflect light, so it cannot be observed directly, but its gravitational influence on stellar orbits and gravitational lensing reveals both its presence and its distribution.
Multiple independent lines of evidence — including galaxy rotation curves, gravitational lensing around galaxy clusters, the cosmic microwave background's fluctuation pattern, and the large-scale structure of the universe — all point to roughly the same total amount of unseen mass, which also does not behave like ordinary (baryonic) matter in nuclear fusion calculations from the Big Bang. This convergence of evidence across very different methods is why dark matter is considered a distinct form of matter rather than simply undetected ordinary matter.
Swiss astronomer Fritz Zwicky first proposed unseen mass in 1933 after finding that galaxies in the Coma galaxy cluster were moving far too fast to be held together by the visible mass alone, coining the term "dunkle Materie" (dark matter). The idea gained wide acceptance only decades later, after Vera Rubin's rotation-curve work in the 1970s provided much stronger, more direct evidence.