In a map of dark matter, clues to galaxies' histories

The map of the universe, which astronomers have been plotting for some 20 years, reveals that the vast numbers of visible galaxies are not randomly arrayed, but rather display a breathtaking structure. Now scientists are detecting structure in the universe's unseen "dark matter" – whose gravity herds stars into galaxies and galaxies into enormous clusters tens of millions of light-years across.

Sunday, a team of astronomers unfolded the first large-scale map of dark matter, one of the cosmos's most enigmatic ingredients. Such maps are vital to understanding how galaxies evolved and gathered themselves into larger structures, researchers say.

To the untrained eye, the map looks a bit like a hiker's topographic map. But to astrophysicists, the 3-D map – which represents a time span dating back to roughly half the age of the universe – is independent evidence confirming their thesis of how the cosmos evolved after the big bang some 13.6 billion years ago.

Filaments of dark matter grow as time passes, the map indicates. Galaxies form within the filaments. Where filaments cross and join, larger amounts of dark matter accumulate – corresponding to galaxy clusters astronomers can see.

The results represent "an exciting new view of the dark universe," according to Eric Linder, an astrophysicist at the Lawrence Berkeley National Laboratory, in Berkeley, Calif. "Most attempts at detecting dark matter have involved a single galaxy or cluster of galaxies," he says. The new map "scans a much wider area of the universe" – a patch roughly equivalent to an area that would be covered by eight full moons corralled into a single part of the sky, as seen from Earth.

Dark matter has puzzled astronomers ever since its existence was first posited nearly 75 years ago. Nobody can see it directly, so astronomers infer its presence by the influence its gravity exerts on other celestial objects. Without it, for example, galaxies would fly apart as they spin, because the mass of a galaxy's visible matter is not sufficient to generate enough gravity to hold the galaxy together.

Moreover, nobody knows what it's made of, although scientists are testing several ideas. But on the list of ingredients that make up the cosmos, dark matter is a big deal: Cosmologists estimate that it accounts for 23 percent of the universe's matter and energy budget. Matter that astronomers can see makes up only 4 percent of the total. Dark energy accounts for the balance.

To locate dark matter on such large scales, the mapping team of astronomers took a page from Albert Einstein's theory of general relativity and used gravity as a lens. One of the theory's successful predictions: Gravity can bend light, similar to the detour a glass lens imposes as light passes through it.

The team, led by Richard Massey at the California Institute of Technology in Pasadena, gathered detailed images of roughly 500,000 galaxies. Gravity from dark matter distorted the shapes of the galaxies, and the team used those distortions to calculate the mass of dark matter between each object and Earth. Using the Hubble Space Telescope, they spent 1,000 hours imaging 575 overlapping patches of sky. Astronomers gathered follow-up information on the individual galaxies and on normal matter using ground-based telescopes and the European Space Agency's XMM-Newton orbiting X-ray telescope. The team presented its results Sunday in an early online edition of the journal Nature and at a press briefing at the winter meeting of the American Astronomical Society in Seattle.

The data covered three onionlike "layers" of distance, so the map gives scientists a glimpse at how changes in dark-matter distribution altered the distribution of visible matter over time.

For example, notes team member Nick Scoville, several of the early dark-matter structures hold galaxies that are in the process of forming clusters. And the ages of many of these galaxies appear to show that galaxies formed from smaller collections of stars. This would favor one recipe for dark matter: so-called cold dark matter, made of fundamental particles that have far larger masses than a proton and that interact only weakly with ordinary matter. Physicists are hunting for some of these directly at particle accelerators around the world.

The new map reinforces what astronomers call the "concordance" view of the universe's matter-energy proportions – drawn from maps of the visible universe, as well as from new maps of the microwave radiation left over from the big bang.

But the new data present puzzles of their own. The map shows some areas where luminous matter and dark matter don't track each other, notes Dr. Linder. This suggests that the idea of dark matter as skeleton for the large-scale structure and distribution of bright matter may break down under certain situations.

"We may need to fine-tune our ideas of how galaxies form," he says.

What is clear is that cosmic cartography is entering yet another new era.