There’s something strange out in space. Scientists know it’s there, but not what it is. We know about visible stars and planets, gas and dust, swirling around our beautiful spiral galaxy. The stars are all rotating about the center. It takes a long time. The last time the Solar System was in this part of its orbit was at the time of the Triassic-Jurassic extinction, 200 million years ago, at the beginning of the 50 million-year reign of the dinosaurs. But the rotation is puzzling. Something more is pulling us in and stopping the galaxy flying apart.
In the center of our galaxy, the Milky Way, is a monster black hole with a mass about four million times the mass of the sun. But there are roughly a hundred billion stars in the galaxy, so that does not do much in holding them all together. There must be something else that has gravity and therefore has mass. We call it “dark matter,” but “invisible matter” would be more accurate.
The first signs of what he called “dunkle Materie” were by the Swiss astronomer Fritz Zwicky in 1933. He studied a cluster of about a thousand galaxies in the constellation Coma Berenices and measured the spread in their speeds. He argued that they would have flown apart long ago had they not been bound together by the gravity of some unknown matter.
Later Vera Rubin and Kent Ford measured the speeds of stars circling around the Andromeda galaxy and reached a similar conclusion. The stars must be held in by the gravity of about five times the known matter. Rubin confirmed this discovery by measuring the rotation of many other spiral galaxies.
We have since found other evidence for dark matter. For example, its gravitational field deflects the light from more distant galaxies and distorts their images, an effect called “gravitational lensing.”
But what is it? All matter is believed to be made of subatomic particles, but none that we have discovered fits the bill. Are they too massive to be produced even at our highest energy accelerators? Or are they much lighter but interact too weakly to produce and detect? Or neither?
As we strive to understand the universe, physicists are rising to this challenge. I do not even have space to mention the “elephant in the room” – the dark energy that is blowing the universe apart and is even more mysterious!
