The missing mass problem
Measure how fast stars orbit at the edge of a spiral galaxy and you get an answer that should not be possible. They move far too quickly for the visible mass to hold them in place.
Rotation curves
In the solar system, outer planets orbit more slowly than inner ones — the pattern you expect when mass sits in the middle. Galaxies do not behave that way. Orbital speed stays roughly flat all the way out, which means mass keeps accumulating well beyond the visible disc.
Converging evidence
The case does not rest on rotation curves alone:
- Gravitational lensing — clusters bend light more strongly than their visible mass allows
- The Bullet Cluster — after a collision, the lensing mass and the hot gas are in different places
- The microwave background — the pattern of fluctuations requires a component that does not interact with light
- Structure formation — without extra mass, galaxies would not have had time to form
The Bullet Cluster is the hardest to argue away. Gas clouds collided and slowed; the bulk of the mass passed straight through, which is what you would expect from something that ignores everything except gravity.
What it might be
Candidates keep failing. Direct-detection experiments have been running for decades in deep mines, and nothing has been found. Modified-gravity theories explain rotation curves elegantly but struggle with clusters and the microwave background.
Twenty-seven per cent of the universe consists of something we can weigh, map, and describe the behaviour of — while having no idea what it is made of.
Cover image: GSFC — Public domain (NASA) (source).