Secrets of the universe

If you drove into a wall made of dark matter, you wouldn't crack a headlight or inflate an airbag. You wouldn't even know it happened.

Dark matter is the most common substance in the universe, and the most mysterious, and it isn't giving up its secrets any time soon.

"It doesn't interact with light. It doesn't interact with normal matter. It doesn't shine, it doesn't reflect light. We can't see it," said Richard Massey, a postdoctoral researcher at the California Institute of Technology.

Undeterred by such obstacles, Massey and a team of astronomers recently created the first-ever large-scale map of dark matter in the universe. Published in the journal Nature last week, the map confirms for the first time science's understanding of how the universe evolved by revealing the existence of structures so gigantic that they span millions of galaxies.

More than 70 years ago, physicists deduced the existence of this invisible dark matter to solve a huge problem: There isn't enough ordinary, or "bright," matter in the universe to explain why stars, galaxies, and galaxy clusters don't fling themselves apart in every direction from the force of the big bang. To hold everything together, you need more gravity. The source of this gravitational glue must be additional matter beyond what can be seen. This is what scientists call dark matter.

Most physicists believe that dark matter is made from some exotic, intangible particles, although there is less agreement as to exactly what these particles might be.

"We really don't know," said Massey. "It's really that mysterious."

Until now, astronomers had only been able to take "snapshots" depicting dark matter surrounding relatively small galaxy clusters. One of those snapshots -- taken by the Chandra X-Ray Observatory space telescope -- was touted by NASA last year as the first direct proof that dark matter exists.

The new map encompasses an area that includes about two million galaxies -- still only a relatively modest slice of the universe.

Since dark matter is invisible and intangible, it must be detected indirectly. Massey and his team did so by targeting the one flaw in dark matter's cloak of invisibility: gravity.

The key is "gravitational lensing," the technical term for when light is distorted by the force of gravity. The distortion is analogous to a heat mirage or looking through a glass shower door.

"The whole universe is slightly wobbly and distorted through this dark matter gravitational lens," said Massey.

By measuring these wobbles in the light emitted by distant galaxy clusters, the researchers estimated the gravitational force causing them, sketching out where the dark matter must be.

To create the map, Massey's team used a battalion of computers to analyze hundreds of overlapping pictures taken by the Hubble Space Telescope.

"In broad outline, the maps confirmed our expectations of how structure arose in the universe," said Edmund Bertschinger, a professor of physics at the Massachusetts Institute of Technology whose study of dark matter helped create those expectations.

Because light takes time to travel, the arrangement of dark matter seen in the map gets older the farther it is from earth, so it provides a window into how the universe is evolving.

In the earliest days after the big bang, all matter was evenly distributed. But there were tiny imbalances at subatomic scales. The rapid expansion of the universe after the big bang magnified these tiny imbalances, said Bertschinger, with gravity causing matter -- both dark and bright -- to congeal around the unbalanced areas.

The dark matter condensed first because its intangible nature makes it frictionless. Ordinary matter was slower to congeal because its momentum was dampened by colliding with itself and with particles of cosmic radiation. But the gravity of dark matter eventually pulled the bright matter along in its wake.

"The normal matter flows gravitationally into this sort of dark matter scaffolding," said Massey, "and is constructed within that into the galaxy and the stars we see today."

The dark matter and its gravity shaped bright matter in a manner loosely reminiscent of how the texture of the ground shapes puddles of rainwater. As the "puddles" started to form in the early universe, they began to exert gravity themselves, making for bigger puddles.

"Gravity is an attractive force. It builds little things into medium-sized things, medium-sized things into big things, and over time it builds up the structures," said Eric Linder, an astrophysicist at the University of California, Berkeley.

Over time, the gravity from the medium-sized and large structures that formed first began pulling on matter from every direction. Since dark matter is frictionless, it was twisted and shaped by gravity alone into increasingly ornate and complex shapes -- clumps, clusters, and what Massey calls "filaments."

The filaments are unimaginably huge -- hundreds of millions of light years long and proportionately wide. And where the filaments run, stars and galaxies congregate in ever denser and more elaborate formations. Where the filaments intersect, massive galaxy clusters form with black holes at the crossroads.

Though more ambitious than any previous work, the new dark matter map still represents only a rough draft of the history of the universe. Massey is already thinking ahead.

"One thing we want to do is start thinking about dark energy, which is sort of a bizarre thing," Massey said -- no small statement considering the piece of work he just completed.

Scientists invented the concept of dark energy, a theoretical source of antigravity, to address another problem: why the universe's expansion is accelerating. Studies of dark energy will be built, in part, on a new generation of dark matter maps as well as on research charting how cosmic radiation is distributed throughout the universe.

Future dark matter maps will encompass larger areas in finer resolutions, allowing astronomers to track the evolution of the universe with more and more detail.

"It's a little bit like the 16th-century maps of the early explorers, the first maps of the continents of the earth," said Bertschinger. "There were some early distortions that improved as more voyages were made, and in a similar manner, this map will improve as more observations are made and we examine a larger area of the sky."

For more views of the map of dark matter, go to: spacetelescope.org/videos/html/mov/320px/heic0701f.html.