The Mystery of Dark Matter
Something holds galaxies together that no instrument has detected directly. After half a century of searching, the leading candidates are being ruled out one by one, and that is progress.
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Quick Read · about 34 seconds
- What happened
- Galaxies rotate too fast, clusters bend light too strongly and the early universe grew structure too quickly for the matter we can see. The inferred missing mass, called dark matter, outweighs ordinary matter about five to one. Decades of direct-detection experiments have found nothing, steadily shrinking the space where the leading candidate particles could hide.
- Why it matters
- Dark matter is most of the matter in the universe. Not knowing what it is means not knowing what most of the universe is made of, and it leaves open the possibility that gravity itself behaves differently than the standard theory says.
- What remains uncertain
- Whether dark matter is a particle at all, and if so which kind; whether modified theories of gravity can explain the evidence instead; and whether current experiments are looking in the right place.
In the 1970s the astronomer Vera Rubin measured how fast stars orbit in spiral galaxies and found something that should not have been true. Stars at the edges moved as fast as stars near the center. Under the gravity of the visible matter alone they should have been slower, and the galaxies should have flown apart. Something unseen was holding them.
Half a century later the evidence has multiplied. Clusters of galaxies bend light from objects behind them more than their visible mass allows. The faint afterglow of the Big Bang carries the imprint of matter that does not interact with light. Simulations of how the universe's large-scale structure formed only match observations when most of the matter is of this invisible kind.
What it is not
The missing mass is not ordinary matter that happens to be dark, such as dim stars or gas. Measurements of the early universe constrain how much ordinary matter exists, and it is not enough. Dark matter is something that has mass, feels gravity, and does not interact with light or with ordinary matter in any way yet detected.
The search
The leading hypothesis for decades was a weakly interacting massive particle, a heavy particle that would occasionally bump into an atomic nucleus. Detectors buried deep underground, shielded from cosmic rays, have waited for such collisions since the 1990s. They have grown enormously more sensitive and have detected nothing. Each null result rules out a range of possible particle masses and interaction strengths.
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Dark matter makes up about five times more of the universe's matter than ordinary matter does.
Fact · confidence high
- Planck 2018 results. VI. · Planck Collaboration
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