Wing V · the map roomRoom II of II
Inv. no. V.2 · the turntable
Dark matter
A galaxy is a turntable of stars. Near the middle, where most of the light is, they should go round fast; further out, with less mass inside their orbit, they should slow down, the way Neptune crawls round the Sun while Mercury races. They don't. Far out in the faint outskirts, stars and gas go round as fast as anywhere. Something heavy is holding them in, and it gives off no light at all.
The turntable
Fig. I. A galaxy like ours from above, 32,000 parsecs across. The dashed ring is the Sun's orbit.
- The Sun goes round at
- What shines would give
- Once round
- Inside the Sun's orbit
Press only what shines and the galaxy runs on the stars and gas we can see. The middle still whirls, but the outskirts fall behind, each ring slower than the one inside it, and the spiral smears out. That is how the solar system works, where nearly all the mass sits in the Sun. It is not what galaxies do.
Now press measured speeds, no dark matter. Every star gets the speed it really has, but only the visible matter pulls on it. The middle holds; the outskirts are going far too fast for so little mass, and they leave. A galaxy that looked like this would come apart in a few hundred million years, and galaxies are billions of years old. So the mass we see cannot be all there is.
The rotation curve
Fig. II. How fast an orbit goes at each distance, built from its parts. The cream line is what stars do; the brass line, what the visible matter alone would make them do.
- Twice as far out as the Sun, at 20 kpc
A rotation curve is measured from the Doppler shift of starlight and of the radio glow of hydrogen gas, which reaches much further out than the stars. Every spiral galaxy measured this way shows the same thing: the curve rises, then stays flat, out to where the gas runs out. The visible matter makes a curve that peaks and falls. The difference is drawn in blue: a halo of matter that pulls but does not shine, spread out far beyond the visible disc.
Slide the dark matter away and the cream line falls back onto the brass one. Slide it up and the outskirts would go faster than anything we see. The model here is a textbook galaxy like ours, tuned so that the Sun goes round at about 230 km/s, as measured.
Who worked it out
Where it already matters
Too fast in Coma, 1933. Fritz Zwicky measured how fast the galaxies in the Coma cluster move and found them far too fast to be held together by the light he could see. He called what was missing dunkle Materie, dark matter. For forty years almost nobody took it up.
Flat curves, 1970s. Vera Rubin and Kent Ford measured the rotation of the Andromeda galaxy, published in 1970, and then of one spiral after another: flat, every time. That made dark matter impossible to ignore. The large survey telescope in Chile that took its first pictures in 2025 carries her name: the Vera C. Rubin Observatory.
The Bullet Cluster, 2006. Two clusters of galaxies that passed through each other. Their hot gas, most of the ordinary matter, collided and stayed in the middle; but the bending of background light shows most of the mass sailing on with the galaxies. Whatever dark matter is, it passed straight through.
The cosmic budget. From the afterglow of the Big Bang, the Planck satellite (2018) puts ordinary matter at about 5% of everything, dark matter at about 27% and dark energy at about 68%. Five parts dark to one part of everything that makes stars, planets and us.
Still unseen. Detectors deep underground have looked for dark matter particles passing through for decades, so far without a catch. A minority of physicists think gravity itself works differently at very low accelerations (MOND, Mordehai Milgrom, 1983) and that there is no dark matter at all; clusters like the Bullet are hard for that idea.