Dark Matter

By Michael Lemonick

Last summer, astronomers used a really clever observing strategy to present the best evidence to date that dark matter, once dismissed as a hare-brained idea, really does make up most of the mass of the universe. The image above does it again, in a slightly different way.

What you're looking at is a giant cluster of galaxies that underwent a collision with another cluster just behind it (we just happen to be on the line of sight that includes both clusters). As the two passed through each other a billion or two years ago, their combined gravity sucked the dark matter in and around the visible galaxies toward the center of the cluster. In the collision's aftermath, the dark matter rebounded, but then slowed and piled up in the bluish ring you can see in the photo.

The ring, and the clump of dark matter you can see in the core of the cluster, aren't really blue. They aren't really visible at all (there's a reason they call it "dark matter.") They're simulations, based on the the fact that gravity distorts the passage of light: the very strong gravity of the dark matter has warped the images of more distant galaxies, and by measuring the amount and type of warping with great precision--something that would have been impossible without the Hubble Space Telescope--the astronomers can infer where and how much dark matter there is.

This simulation shows not only that it's there, but also, because it's clumped in ways the ordinary matter is not, that it's not made of some dark version ordinary matter--confirming what theorists have been saying for years.

The phenomenon that makes this all possible, by the way, known as gravitational lensing, was discussed by Einstein in a celebrated paper back in the 1930s. His conclusion, after explaining how it would work: we'll probably never actually observe this. The first gravitational lens was discovered in 1979, and lensing has since become one of the most powerful tools for probing the universe. For once, Einstein just didn't think imaginatively enough.