Dark matter is one of the biggest unsolved mysteries in science, and it is also one of the most solidly established. Astronomers are confident it exists, even though nobody has ever seen it directly, because galaxies behave in ways that make no sense without it. Stars at the edges of spiral galaxies orbit far too fast: by the gravity of the visible stars and gas alone, they should be flung off into space, yet they stay put.

The name is a little misleading. Dark matter is not dark in the sense of being black or shadowy; it simply does not seem to interact with light at all. It does not emit light, absorb it or reflect it, which is why telescopes cannot see it. The only way we know it is there is through its gravity, which bends the paths of stars and light and holds galaxies and clusters of galaxies together against their own spin.

The evidence: spinning galaxies

The first strong hint came in the 1970s, when astronomer Vera Rubin measured how fast stars orbit at different distances from the centres of spiral galaxies. If galaxies contained only the stars and gas we can see, the orbital speeds should drop off with distance, just as the outer planets orbit the Sun more slowly than the inner ones. Instead, Rubin found that the speeds stay roughly flat all the way out to the edge of the visible disc and beyond.

The evidence has only grown stronger since. Galaxies in clusters move too fast to be held together by their visible mass. The light from distant galaxies is bent by the gravity of intervening clusters far more than the visible matter in those clusters could manage, an effect called gravitational lensing that lets astronomers map where the unseen mass actually sits. And the pattern of temperature ripples in the cosmic microwave background, the afterglow of the Big Bang, only matches what we observe if about 27 percent of the universe is dark matter, with ordinary matter making up less than 5 percent.

What could it be made of?

Nobody knows, but there are leading candidates. One idea is that dark matter is made of WIMPs, weakly interacting massive particles, a hypothetical type of particle that would rarely collide with ordinary matter and would pass through the Earth almost unnoticed. Enormous detectors buried deep underground, shielded from cosmic rays, have been searching for the rare flash of a WIMP hitting an atomic nucleus for decades, without a confirmed detection so far. Another candidate is the axion, an extremely light particle proposed for unrelated reasons in particle physics, which would behave more like a wave spread across space.

There are also more exotic possibilities, including tiny primordial black holes formed in the first moments after the Big Bang, or entirely new kinds of particles nobody has yet imagined. Some physicists have proposed instead that our theory of gravity itself needs modifying at galactic scales, an idea known as MOND, but modified gravity struggles to explain the full range of evidence, especially the lensing maps and the cosmic microwave background, which dark matter explains naturally. For now, the particle explanation remains the mainstream view.

Why it matters for the universe we see

Dark matter did more than hold today's galaxies together; it built them in the first place. In the early universe, ordinary matter was too hot and too spread out to collapse into galaxies on its own. Dark matter, immune to the pressure of light, could start clumping together first, forming a scaffolding of invisible halos. Ordinary gas then fell into those gravitational wells, cooled, and ignited into the first stars and galaxies. Without dark matter, the universe would likely be a thin, nearly featureless soup of hydrogen and helium, with no galaxies, no stars, no planets and nobody to wonder about any of it.

The search continues on three fronts: giant underground detectors watching for particles, telescopes mapping dark matter through gravitational lensing, and particle colliders trying to create dark matter particles in the laboratory. Any one of those could crack the mystery, and the discovery would rank among the greatest in the history of science. Until then, dark matter remains a humbling reminder: the stuff we are made of, and everything we have ever seen in the sky, is only a small fraction of what the universe actually contains.

Frequently Asked Questions

Q: Is dark matter the same as dark energy?

A: No. Dark matter is invisible mass that pulls things together through gravity and helped form galaxies. Dark energy is a separate, even more mysterious effect that is pushing the expansion of the universe to accelerate.

Q: Could dark matter just be ordinary matter we cannot see, like dim stars or black holes?

A: Astronomers have checked. There are not nearly enough dim stars, gas clouds or black holes to supply the missing mass, and the cosmic microwave background rules out ordinary matter as the main ingredient.

Q: Has anyone ever detected a dark matter particle?

A: Not yet. Underground detectors and colliders have searched for decades without a confirmed signal, which is why the hunt now includes lighter particles such as axions as well as WIMPs.