The basic idea
Astronomers measure mass by watching how things move. If something orbits under gravity, its speed tells you how much mass is pulling on it. When this method is applied to galaxies and clusters of galaxies, the result is that there seems to be much more mass than the stars, gas and dust we can see. The extra, unseen mass is called dark matter. The word “dark” means it does not appear to emit, absorb or reflect light in any way we can detect, not that it is a black cloud.
It is better to think of dark matter as an inference than as an observation. Nobody has directly detected it in a laboratory. What astronomers see is a repeated mismatch between visible matter and gravitational effects.
Early hints from galaxy clusters
In the 1930s, Fritz Zwicky studied the motions of galaxies in a large cluster. The galaxies moved so fast that, by his estimate, the visible matter should not have been enough to hold the cluster together. He suggested there must be unseen mass. His numbers were later revised, but the basic puzzle has remained.
Rotation curves
The most widely cited evidence comes from how stars orbit within spiral galaxies. In our own solar system, planets far from the Sun move more slowly than those close in, because most of the mass is at the center. If a galaxy’s mass were concentrated where its light is, stars on the outskirts would be expected to orbit more slowly too.
In the 1970s, Vera Rubin and colleagues measured the speeds of stars and gas at different distances from galactic centers and found that the speeds stay roughly flat far out, instead of dropping off. That suggests there is additional mass spread through and around the galaxy, in a large halo extending well beyond the visible disk.
Gravitational lensing
Massive objects bend the path of passing light, an effect predicted by Einstein’s general relativity. A cluster of galaxies can act like a weak lens, stretching and distorting the images of more distant galaxies behind it. By analyzing the distortion, astronomers can map where the mass is. In many clusters, the mapped mass is considerably larger than the visible matter would explain. This method does not rely on measuring motion, so it gives an independent check, using the same gravitational force in a different way.
The Bullet Cluster
One famous system, the Bullet Cluster, consists of two galaxy clusters that have collided. The hot gas, which makes up most of the ordinary matter, was slowed by the collision and lies near the middle. Lensing maps, however, show that most of the mass has moved on past the gas, separate from it. This is often presented as strong evidence that the dominant mass is not ordinary gas, and that it interacts very weakly, if at all, with itself and with normal matter.
Evidence from the early universe
The faint glow left over from the early universe has tiny patterns of temperature variation. The details of these patterns depend on how much ordinary matter and how much other matter existed at the time. Models that fit the patterns require a substantial component of non-ordinary matter. Similarly, computer simulations of how galaxies and clusters grow work best when dark matter is included, because it helps clumps of mass form early. These lines of evidence reinforce each other, which is one reason most astronomers regard the dark matter hypothesis as well supported.
What could it be?
Several candidates have been proposed.
- New particles. Many physicists suspect dark matter consists of particles not included in the established particle physics model. Weakly interacting massive particles and very light particles called axions are among the best-known ideas.
- Ordinary but dim objects. Faint stars, black holes or planets were once suggested, but surveys suggest they cannot account for most of the missing mass.
- Modified gravity. Some researchers propose that our laws of gravity need adjusting at very low accelerations, which would remove the need for unseen matter. These models can reproduce some galaxy rotation patterns but have generally struggled to explain all the evidence, such as cluster collisions, with a single approach.
Experiments built deep underground, particle colliders and space-based detectors have searched for evidence of dark matter particles. So far, no confirmed detection has been announced. That does not disprove the hypothesis; it may mean the particles interact even more weakly than expected, or that they are a different kind than the one being sought.
How much is there?
In the standard cosmological model, dark matter makes up roughly a quarter of the universe’s total energy content, and about five times as much as ordinary matter by mass. These figures are approximate. It should not be confused with dark energy, which is a separate idea connected to the accelerating expansion of the universe.
What remains unknown
Open questions include what dark matter actually is, whether it consists of one kind of particle or several, whether it has any interactions beyond gravity, and how it behaves on the smallest scales inside dwarf galaxies. Astronomers also work on whether the apparent flaws in simulations at small scales come from missing physics or from the complicated behavior of ordinary gas.
For a reader new to this subject, a helpful starting point is astronomy basics, then the way cosmology uses these ideas. The central message is simple: gravity tells us something is there, and scientists are still working to learn what it is.