Most of the universe is missing. When astronomers add up all the stars, gas, and galaxies they can see, the total falls far short of what gravity says should be there. To explain the difference, scientists invoke something invisible that pulls on everything but gives off no light at all. They call it dark matter, and a century after the first hints, no one is certain what it is.

What dark matter is

Dark matter is a form of matter that exerts gravity but does not emit, absorb, or reflect light. That makes it invisible to every kind of telescope, since telescopes detect light. We know it through its gravitational pull on the things we can see.

It is not the same as dark energy, a separate mystery thought to drive the accelerating expansion of the universe. Dark matter pulls things together; dark energy pushes the cosmos apart. Estimates suggest ordinary matter, the kind that makes up stars, planets, and people, is only a small fraction of the universe, with dark matter several times more abundant.

The evidence: galaxy rotation curves

The first strong clue came from watching how galaxies spin. In a galaxy, stars orbit the center much as planets orbit the Sun. With the Sun, the planets farther out move more slowly, because most of the mass is concentrated at the center.

Astronomers expected the outer stars of a galaxy to behave the same way and slow down. Instead, they found that stars far from a galaxy's center orbit just as fast as those closer in. The visible matter simply cannot supply enough gravity to hold those fast-moving outer stars in place; they should fly off.

The most widely accepted explanation is that each galaxy sits inside a vast, invisible halo of dark matter whose extra gravity keeps the outer stars bound. The shape of these rotation curves became one of the cornerstones of the case for dark matter.

The evidence: gravitational lensing

A second, independent line of evidence comes from light itself. According to Einstein's theory of gravity, mass bends the path of light passing nearby. A massive object can therefore act like a lens, distorting and magnifying the image of more distant galaxies behind it. This is called gravitational lensing.

When astronomers measure how strongly clusters of galaxies bend the light of background objects, the bending is far greater than the visible matter could produce. The extra lensing implies a large amount of unseen mass. In some colliding galaxy clusters, the inferred mass even appears to be separated from the visible gas, which many researchers regard as especially direct evidence for dark matter.

Wherever astronomers weigh the universe by its gravity, they keep finding more mass than they can see.

Other supporting clues

Dark matter also helps explain larger patterns:

  • The structure of the cosmos. Galaxies are arranged in an enormous web of filaments and voids. Computer simulations reproduce this structure well only when dark matter is included to provide the gravitational scaffolding on which ordinary matter gathers.
  • The afterglow of the Big Bang. Tiny variations in the cosmic microwave background, the leftover radiation from the early universe, fit best with models that contain a substantial amount of dark matter.

Leading candidate ideas

If dark matter is real, what is it made of? The favored idea is that it consists of some new kind of subatomic particle not included in the current standard model of physics, one that interacts mainly through gravity and perhaps the weak nuclear force. Several hypothetical particles have been proposed as candidates. Other researchers explore the possibility that our understanding of gravity itself is incomplete on the largest scales, though this faces difficulty explaining all the observations at once.

Still undetected

Here is the crucial caveat: dark matter has never been directly detected. Sensitive experiments, often placed deep underground to block interference, have searched for the faint signal of a dark matter particle striking ordinary matter. Particle accelerators have looked for it as well. So far, none has found it. Every piece of evidence we have remains indirect, inferred from gravity rather than caught in a detector.

The bottom line

Dark matter is the name for an invisible mass that appears to pervade the universe, revealed by the fast spin of galaxies, the bending of light around galaxy clusters, and the structure of the cosmos itself. It seems to outweigh ordinary matter several times over, yet no experiment has ever detected it directly. It is one of the largest open questions in science: we can measure its pull everywhere, and still cannot say what it is.