Dark Matter: Evidence and the Identification Gap

Vera Rubin and Kent Ford spent the 1970s at the Carnegie Institution of Washington mapping the rotation of dozens of spiral galaxies. Newton's gravity made a clear prediction: just as the outer planets orbit the Sun more slowly than the inner ones, stars far from a galaxy's centre should circle more slowly than those near it. The observation was something else. The rotation Rubin measured stayed just as fast far beyond the visible disk, in galaxy after galaxy. Either Newton's gravity fails at galactic scales, or galaxies hold far more matter than we can see — five to ten times more, spread through an invisible halo. The second reading is the mainstream account. The ordinary matter that makes up stars, planets, and us — baryonic matter — accounts for only about 5% of the universe's energy, and the bulk of the gravitating universe is some form of matter we have never directly detected.

The case for cold dark matter — 'cold' meaning it moves slowly compared with light — rests on four largely independent lines of evidence. First, galaxy rotation curves, Rubin's program: the gravitating mass of any large galaxy reaches far beyond its visible edge. Second, gravitational lensing — the bending of light from background galaxies by the mass of a cluster in front of them — which reveals several times more mass than the cluster's visible matter holds. The cleanest case is the Bullet Cluster, two clusters caught mid-collision: the visible hot gas has been dragged to the middle, but the bulk of the mass (mapped by its lensing) has sailed straight through and sits to either side — almost impossible to explain by tinkering with gravity. Third, structure formation: simulations of how galaxies assemble match the real universe only if cold dark matter is included, which is the heart of the standard ΛCDM model (dark energy plus cold dark matter plus ordinary matter). Fourth, the cosmic microwave background — the afterglow of the hot early universe — whose precise pattern of ripples, measured by Planck, fits a universe with dark matter and rules out one made of ordinary matter alone. Theories that instead modify gravity can fit some of this but founder on the Bullet Cluster and the CMB. So the mainstream view is that dark matter is real — we simply have not identified the particle. The leading suspects are WIMPs (heavy, feebly interacting particles suggested by some extensions of the Standard Model) and axions (extremely light particles proposed to fix an unrelated puzzle in the physics of the strong force); primordial black holes are a third. Decades of ever-more-sensitive underground experiments — LZ, XENONnT, PandaX — have not caught a WIMP; dedicated experiments hunt the axion; and gamma-ray and neutrino observatories add further constraints. That the evidence for dark matter is overwhelming while direct detection stands at zero is itself a powerful clue to what it can be.