AUSTIN, TEXAS-Astronomers have made the largest map yet of dark matter in the universe. This invisible stuff gives off no light, but it does exert gravity on its surroundings. It probably consists of unknown elementary particles, and it's much more prevalent than the normal matter from which stars, planets, and people are made.
The new map shows that dark matter is concentrated in huge clumps and filaments, with giant, empty regions in between—just as computer simulations had predicted. "We're very happy to see that our results are similar to what we expected," says astrophysicist Ludovic Van Waerbeke of the University of British Columbia, Vancouver, in Canada.
Mapping the invisible may sound impossible, but in fact it's rather simple. Just as an invisible man sleeping in your bed will leave wrinkles in the sheets, the gravity of invisible dark matter produces minute distortions in the observed shapes of background galaxies. Using this "weak lensing" effect to map dark matter is "a first important step to understand the dark Universe," says Van Waerbeke's co-worker Catherine Heymans of the University of Edinburgh in the United Kingdom.
Working with the 340-megapixel MegaCam camera on the 3.6-meter Canada-France-Hawaii Telescope (CFHT) on Mauna Kea, Hawaii, the team spent 5 years imaging 10 million galaxies at distances of about 6 billion light-years. "Our map is about a hundred times larger than the largest dark matter map to date," Van Waerbeke says. A statistical analysis of the shapes of the galaxies revealed the spatial distribution of the intervening dark matter.
The results, presented here at the 219th meeting of the American Astronomical Society, look very much like supercomputer simulations of the evolution of the universe, with dark matter clumped into a "cosmic web" of filaments and knots. The clumpy knots, where most of the dark matter is concentrated, neatly coincide with huge clusters of galaxies, just as cosmological theories suggest.
In fact, says astrophysicist Rachel Mandelbaum of Princeton University, "projects like the CFHT Lensing Survey can be used to test theories of dark matter and general relativity." So far, Van Waerbeke says, "everything looks okay. The maps show exactly what we expected." In other words, the results confirm current popular ideas about the physics, makeup, and evolution of the universe.
According to team member Fergus Simpson of the University of Edinburgh, the lensing survey shows not only how dark matter bends light but also how it clumps together over time. The results, he says, already rule out a number of suggested alternatives to Einstein's general theory of relativity. For instance, Simpson says, a theory known as Modified Newtonian Dynamics  is no longer supported by the dark matter data.
Whereas lensing surveys reveal the distribution of dark matter, other types of observations planned for the near future by other projects will shed light on the even more mysterious dark energy that appears to accelerate the expansion of the universe, Heymans explains. A future European space telescope called Euclid will carry out these two types of observations simultaneously, she says. "We really need both."