In 1933 a Swiss astronomer based at Caltech, Fritz Zwicky, pointed his attention at the Coma Cluster, a swarm of galaxies some 300 million light years away, and did a piece of cosmic accountancy that has troubled physics ever since. He measured how fast the galaxies were flying around within the cluster and reasoned, quite simply, that gravity must be holding them together — otherwise they would long ago have scattered into space. From their speeds he could calculate how much mass was needed to keep them bound. Then he added up the mass he could actually see, in the form of glowing stars and galaxies. The two figures did not remotely match. The cluster appeared to contain far more mass than its visible contents could account for — by a wide margin.
Zwicky's conclusion was startling: the cluster must be full of matter that emits no light at all. He called it "dunkle Materie" — dark matter. It was one of the great insights of twentieth-century astronomy, and it was almost entirely ignored for decades.
An unwelcome messenger
Part of the problem was Zwicky himself. Brilliant and wildly inventive — he coined the term "supernova" and correctly anticipated neutron stars and gravitational lensing years ahead of their time — he was also famously abrasive, feuding with colleagues he liked to call "spherical bastards", because, he said, they were bastards whichever way you looked at them. His measurements were rough, the distances of the era were poorly known, and the idea of vast quantities of invisible matter seemed too outlandish to pursue. For roughly forty years the missing-mass problem sat unresolved at the edge of astronomy.
The evidence that finally forced the issue came from a very different scientist. In the 1970s the American astronomer Vera Rubin, working with the instrument-builder Kent Ford, set out to measure how stars orbit within individual spiral galaxies. Basic physics predicted that stars far from the bright, crowded centre of a galaxy — where most of the visible mass lies — should orbit more slowly, just as the outer planets circle the Sun more slowly than the inner ones. That is not what Rubin found. The outer stars were moving just as fast as the inner ones. The galaxies' rotation curves, as they are called, stayed stubbornly flat all the way out.
Flat curves and a cosmic ledger
There was only one straightforward explanation. Each galaxy had to be embedded in a vast, invisible halo of extra mass, extending well beyond the visible starlight, whose gravity was speeding up the outer stars. Rubin's careful, repeatable observations across many galaxies were far harder to dismiss than Zwicky's single cluster, and they steadily convinced the astronomical community that most of the matter in the universe is dark. Rubin, who spent much of her career pressing for the inclusion of women in astronomy, never received the Nobel Prize — an omission many colleagues consider one of the field's clearest injustices.
Since then the evidence has piled up from every direction: the way clusters of galaxies bend passing light through gravitational lensing, the pattern of faint radiation left over from the Big Bang, and the very way galaxies formed and clumped together in the early universe. All point to the same conclusion. Ordinary matter — everything made of atoms, every star, planet, person and pebble — makes up only about a sixth of the total. Roughly 85 per cent of all the matter in the universe is dark, of a kind we have never directly detected and do not understand. Whatever it is, it is not made of the protons and neutrons that constitute everything familiar; it appears to be some entirely new form of matter, passing straight through ordinary material and interacting with it, so far as anyone can tell, only through gravity.
There are dissenting voices. A minority of physicists suspect the problem lies not in missing matter but in our understanding of gravity itself, proposing that Newton's and Einstein's laws might behave differently on the vast scales of galaxies. Such theories have struggled to explain all the evidence at once, and the dark-matter picture remains the strong consensus. But the honest position is that the leading explanation for most of the universe's mass is a substance no experiment has ever held.
Physicists have hunted for the stuff for decades in deep underground laboratories and vast particle detectors, searching for the exotic particles it might be made of. So far it has refused to show itself in anything but its gravitational pull. We know it is there because of how it moves the things we can see — the same argument Zwicky made, staring at the Coma Cluster, nearly a century ago.
It remains one of the most humbling facts in science: we have mapped the heavens in exquisite detail, yet most of what holds them together is a substance we cannot see, cannot catch, and cannot name with any confidence.
Quiz nuggets
- Fritz Zwicky coined the term "dark matter" ("dunkle Materie") in 1933 after studying the Coma Cluster of galaxies.
- Zwicky also coined the word "supernova" and anticipated neutron stars and gravitational lensing.
- In the 1970s Vera Rubin, working with Kent Ford, found that spiral galaxies have flat rotation curves, strong evidence for dark matter.
- Roughly 85 per cent of all matter in the universe is thought to be dark matter, undetectable by light.
- Dark matter has only ever been observed through its gravitational effects, never directly detected.