Everything you have ever seen, every star, planet and person, is about 5% of the universe. The rest has never been directly detected, and we know it is there only by what it does.
“Dark” here does not mean black. It means it neither gives off nor absorbs light, so we can not see it. We know about it for one reason: its gravity. The story is a detective story with a few big clues, and some of the clues are still open.
1933. The Swiss astronomer Fritz Zwicky measured the speeds of galaxies in the Coma Cluster. They were moving much too fast for the cluster to stay together given the mass of its visible stars. He concluded that there had to be a large amount of unseen matter, and called it dunkle Materie, dark matter. Few took it seriously.
1970s. Vera Rubin and Kent Ford measured how fast stars and gas orbit in spiral galaxies, starting with Andromeda. The result was the same in galaxy after galaxy: a flat rotation curve (a graph of orbital speed against distance from the centre). Rubin, who in 1965 had been the first woman officially permitted to observe at the Palomar Observatory, made the case so carefully that the field had to accept it. She never received a Nobel Prize. In 2025 the world's most powerful survey telescope, the Vera C. Rubin Observatory in Chile, with a 3.2-gigapixel camera, began its work in her name.
2006. The Bullet Cluster is two galaxy clusters that have collided and passed through each other. Most of the ordinary matter is hot gas, and the gas of the two clusters crashed into each other and stayed in the middle. But maps of where the mass is, made using gravitational lensing (explained in Part 2), show the mass passed straight through, to the outsides. The mass is where the visible gas is not.
A mass bends light. Einstein predicted it in 1915 and in 1919 Arthur Eddington led an expedition to photograph stars near the Sun during a total solar eclipse. The stars appeared displaced, by just the amount Einstein had predicted. It made Einstein world-famous overnight.
A big enough mass, such as a galaxy cluster, acts like a lens. If a distant galaxy lies behind it, you see arcs, multiple images, even a perfect Einstein ring. The amount of bending tells you the total mass of the lens, whether it shines or not. When astronomers do this they find far more mass than the stars and gas account for.
| Candidate | Idea | Status |
|---|---|---|
| Ordinary dark objects (faint stars, planets, black holes) | Just hard to see | Ruled out as the main answer by microlensing surveys and by the cosmic microwave background (the afterglow of the Big Bang from lesson 3): there is not enough ordinary matter in total. |
| WIMPs | Heavy particles that barely interact with anything | Huge underground detectors have searched for decades and found nothing so far. |
| Axions | Extremely light particles | Experiments are underway. Nothing detected. |
| Primordial black holes | Tiny black holes made in the Big Bang | Constrained, but not fully ruled out in some mass ranges. |
| Modified gravity (MOND) | No dark matter: gravity itself works differently where forces are extremely weak | Fits galaxy rotation curves well. Struggles to explain clusters, the CMB and the Bullet Cluster. |
Dark matter is a very good description: it explains rotation curves, lensing, the CMB patterns and the way galaxies cluster in a cosmic web. But nobody has found the particle. That is a genuine open problem, and a young physicist could be the one who solves it.
By the 1990s, astronomers expected the expansion of the universe to be slowing, because all the matter in it pulls everything together. Two teams set out to measure how much. They used Type Ia supernovae, white dwarfs that explode at nearly the same brightness every time, as “standard candles”: objects whose true brightness is known, so how dim they look tells you how far away they are. In 1998 both teams reported the same shocking answer: the distant supernovae were dimmer than expected, so farther away than they should be. The expansion of the universe is speeding up. Saul Perlmutter, Brian Schmidt and Adam Riess shared the 2011 Nobel Prize.
Whatever drives that is called dark energy. The simplest candidate is a constant energy of empty space itself, the cosmological constant Λ, which Einstein added to his equations in 1917 and later abandoned. A story says he called it his biggest mistake. That quotation is second-hand, and may be a legend. Quantum theory predicts a value for the energy of empty space that is wrong by a huge factor, by one estimate about 10120, which is sometimes called the worst prediction in physics.
How does it end? If dark energy stays constant, galaxies drift apart forever and stars burn out: a cold, dark Big Freeze. Use the lab to explore the other options.
The white dots are measured orbital speeds for a spiral galaxy at different distances from its centre. The grey curve is what the visible stars and gas alone would give. Add a halo of dark matter with the slider until the pink curve passes through the dots.
A distant blue galaxy lies behind a massive foreground galaxy (gold). Move the background galaxy until it lines up behind the lens. Make the lens heavier and the ring gets bigger.
Five questions.
A sceptic says: “Dark matter is just a fudge factor. You invented invisible stuff to make the equations work.” Give the best answer you can. Use at least two independent kinds of evidence (rotation curves, lensing, the Bullet Cluster, the CMB), and say what would change your mind.