In the 1970s, the astronomer Vera Rubin measured how stars rotate around the centers of galaxies. The result was strange. Stars at the edge of a galaxy moved at almost the same speed as stars near the center. This violates basic physics. By gravity alone, distant stars should orbit slower than closer ones — the way Neptune orbits slower than Mercury.

Either gravity worked differently in galaxies, or there was a lot of mass we couldn't see.

Independent evidence stacked up. Galaxy clusters move too fast given their visible mass. Background galaxies appear distorted by something heavy in the foreground that emits no light. The cosmic microwave background — the afterglow of the Big Bang — has temperature ripples that match a universe full of an unseen substance.

The conclusion is uncomfortable. Roughly 27% of the universe is matter that has gravity but does not interact with light. It does not absorb. It does not reflect. It does not glow. We call it dark matter not because it's black, but because it is invisible at every wavelength we can measure. The visible matter you and I are made of — protons, neutrons, electrons — is about 5% of everything.

What is dark matter? Nobody knows. The leading candidates are heavy unknown particles called WIMPs, axions, or sterile neutrinos. Underground laboratories around the world — at the bottom of mineshafts, shielded from cosmic rays — wait for one to bump into a detector. None has, in over thirty years of trying.

We have charted the gravitational footprint of dark matter across the universe. We have not seen a single particle.