Seeing the Dark
Most of us think of the universe as all the matter there is, and by matter we mean the stuff we can see from afar or could touch if it were up close. But the motion of the observable objects in the universe, like stars and galaxies and clouds of gas, make no sense if the universe contains only ordinary, perceptible matter. This became apparent in 1933, thanks to an astronomer named Fritz Zwicky.
He discovered that parts of a distant cluster of galaxies were moving too fast to remain within the cluster if it contained only ordinary matter. He concluded that dark matter, a phrase he coined, held the cluster together. But for 70-plus years, no one had observed dark matter. It would be like seeing gravity. That has now changed.
Astronomers using ground-based telescopes and satellite observatories have witnessed a separation between visible matter and the dark matter that shapes its motions. It occurred 100 million years ago when two galaxy clusters three billion light-years away passed through each other at about 10 million miles an hour.
Imagine two crowds of pedestrians on a collision course. Some people in both groups — no doubt dressed in black — basically refuse to engage with anyone and just keep moving. But the ordinary people want to stop and chat. As the two crowds merge and then head in opposite directions, the people in black will have pushed ahead, separating themselves from the rest. That, in a nutshell, is what the astronomers saw, minus the people, of course.
Observing what was predicted a lifetime ago is an extraordinary accomplishment. It confirms that this part of our picture of the universe is essentially correct. But observing dark matter and knowing what it is are very different, and we are nowhere near the latter. Then, beyond the problem of dark matter lies the greater problem of dark energy. This is a mysterious universe, and the more we know about it the more mysterious it seems.