Dark Matter

Dark matter is a mysterious form of matter that we cannot see, but which we are absolutely sure exists. It does not give off light, absorb light or reflect light, so no telescope can spot it directly. We know it is there because its gravity pulls on stars, galaxies and even light itself. Dark matter outnumbers ordinary matter (atoms, you, planets, stars) by about five to one. The universe is mostly made of stuff we have never seen.

  • % of universeapprox. 27%Compared to approx. 5% ordinary matter
  • Effect we seeExtra gravityIn galaxies, clusters and light bending
  • Detected byVera Rubin1970s, from galaxy rotation curves
  • Direct detectionNone yetDespite many sensitive experiments
  • Leading candidateUnknown particlePossibly a WIMP or axion
  • Outweighs normal matter5 to 1For every kg of atoms there are approx. 5 kg of dark matter

What is dark matter?

Honest answer: nobody knows. Dark matter is whatever invisible stuff is producing the extra gravity we can detect, but the laws of physics as we currently understand them do not give us a candidate. It is not just ordinary matter that happens to be cold or dark (we have tested for that); it is not made of the same atoms, electrons and protons everything around you is made of.

Whatever it is, dark matter has three main properties: it has mass (because it has gravity), it does not interact with light (so we cannot see it directly), and it interacts very weakly with ordinary matter (which is why it can drift right through Earth without us noticing). Trillions of dark matter particles are probably passing through your body every second without leaving a trace.

How dark matter was discovered

The first hint came in 1933, when the Swiss astronomer Fritz Zwicky studied a cluster of galaxies called the Coma Cluster. He noticed that the galaxies were moving so fast around the cluster that the visible matter's gravity should not be able to hold them in: the cluster should be flying apart. Zwicky suggested there must be some kind of invisible "dunkle Materie" (German for "dark matter") providing extra gravity. He was largely ignored for decades.

The smoking gun came in the 1970s, from the American astronomer Vera Rubin. Rubin carefully measured how stars in spiral galaxies (starting with Andromeda) rotate around the galactic centre. The stars at the outer edge of every galaxy she looked at were going far faster than they should be, given how much visible matter the galaxy contained. The galaxies should have been flying apart. Yet they clearly were not. There had to be something there providing extra gravity: a giant halo of invisible matter around every spiral galaxy.

The evidence keeps growing

Today there are at least half a dozen completely separate lines of evidence for dark matter, all pointing in the same direction.

  • Galaxy rotation curves: stars on the edges of spiral galaxies spin too fast.
  • Galaxy clusters: galaxies in clusters move too fast for the visible mass to hold them.
  • Gravitational lensing: galaxy clusters bend the light from distant objects more than visible matter could.
  • Cosmic microwave background: the pattern of ripples in the CMB exactly fits a universe containing about 27% dark matter.
  • Large-scale structure: the cosmic web of galaxies could not have formed without dark matter providing the initial gravitational scaffolding.
  • The Bullet Cluster: an X-ray and gravity image of two colliding galaxy clusters where the dark matter is clearly visible as a separate component from the gas.
Fact Dark matter is more important than ordinary matter for the structure of the universe. Computer simulations that include the right amount of dark matter produce a perfect match to the cosmic web we see: long filaments of galaxies, big clusters at the junctions, vast empty voids in between. Simulations that leave out dark matter produce a universe that looks nothing like ours.

The Bullet Cluster: visible proof of dark matter

One of the most striking pieces of evidence is the Bullet Cluster: a pair of giant galaxy clusters that crashed into each other about 150 million years ago. In a 2006 image combining X-rays (showing where the hot gas is) and gravitational lensing (showing where the mass is), the gas and the mass are clearly in different places.

The hot gas of the two clusters slowed each other down as they collided, like two clouds of fog blowing through each other. But the mass passed straight through unaffected, because dark matter does not stick to itself the way ordinary matter does. The result is two big clumps of mass on either side, with the gas trapped in the middle. There is no way to explain this without dark matter being a real, physically separate substance from ordinary matter.

What could dark matter be?

Physicists have several candidates for what dark matter might actually be:

  • WIMPs (Weakly Interacting Massive Particles): heavy particles predicted by some theories of physics. Many experiments have hunted for them with no success so far.
  • Axions: extremely lightweight particles also predicted by physics, currently being searched for in several experiments.
  • Primordial black holes: small black holes that formed during the Big Bang.
  • Sterile neutrinos: a heavier cousin of the regular neutrino.

None of these has been found yet. Some of the most sensitive experiments ever built are running right now in deep underground laboratories, hoping to catch the occasional dark matter particle bouncing off an ordinary atom. So far, no clear detection.

Did you know? If dark matter does turn out to be a new kind of particle, every cubic metre of the room you are in probably contains millions of dark matter particles drifting through right now. They mostly do not interact with anything, so they pass straight through Earth (and you) without doing anything at all. The Earth is sweeping through dark matter at hundreds of kilometres per second, but you would never know.
Deeper dive: could "modified gravity" replace dark matter?

Not everyone is convinced dark matter is real. A small group of physicists thinks the rotation of galaxies and the motion of clusters might be explained by a change to our theory of gravity instead, with no need for any invisible matter.

The most famous of these ideas is MOND (Modified Newtonian Dynamics), proposed by Israeli physicist Mordehai Milgrom in 1983. MOND adjusts the law of gravity at very low accelerations (the kind that act on stars far from the centres of galaxies). With this single tweak, MOND can explain the flat rotation curves of spiral galaxies remarkably well, without needing any dark matter.

But MOND struggles to explain everything else dark matter explains. It does not naturally produce the cosmic microwave background pattern. It does not account for the Bullet Cluster, where the gravity is clearly separated from the visible matter. It does not explain how galaxies first formed in the early universe. So while modified gravity continues to be an active research area, almost all astronomers today think dark matter (some new kind of particle) is by far the best explanation for everything we see.

The big question for the next decade is whether one of the dark matter detection experiments finally finds a real particle. If they do, modern physics has a brand new chapter. If they do not, the search continues.

For the other mystery making up most of the universe, see dark energy. For the bigger story, see the Big Bang and the expanding universe.