How do we know something we can't see makes up most of the matter in the universe?

We know dark matter is there because of its gravity. Stars at the edges of spiral galaxies orbit far too fast for the visible stars and gas to hold on to them, and whole clusters of galaxies move as if they weighed much more than everything that shines inside them. Something unseen is adding the extra pull, and by current estimates that invisible something makes up about 85% of all the mass in the universe.

The first person to give it a name was the Swiss astronomer Fritz Zwicky. In 1933 he studied the Coma Cluster and found its galaxies racing around so fast that the cluster seemed to hold about 400 times more mass than he could see. He called the missing stuff dunkle Materie, dark matter. His numbers were far off, but his conclusion held: most of the matter was dark. Decades later, Vera Rubin measured how spiral galaxies spin and found that their outermost stars move as fast as those near the centre. By rights, those galaxies should fly apart. They don't, so a large amount of unseen mass must be holding them together.

Dark matter does not interact with light at all, so no telescope can see it directly. But its gravity bends the light of galaxies behind it, an effect called gravitational lensing, and mapping those tiny distortions shows where the dark matter sits. In the Bullet Cluster, two colliding clusters of galaxies, lensing puts most of the mass in a different place from the hot, glowing gas.

We owe it our existence, too. In the young universe, radiation smoothed out the ripples in ordinary matter, but dark matter ignored the radiation, clumped first and pulled ordinary matter in, speeding up the birth of galaxies. What it actually is remains a mystery: most physicists suspect an undiscovered particle, but no detector has caught one yet.