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SEATTLE - The most detailed 3-D map of the universe ever made, stretching back over billions of years, provides the best evidence yet that mysterious "dark matter" serves as the unseen scaffolding on which everything we can see is hung, astronomers reported Sunday.

The findings are based on years' worth of study, using data from space observatories as well as ground telescopes. But as impressive as those findings are, they still don't tell us exactly what dark matter is made of.

Astronomers from the Cosmic Evolution Survey, also known as COSMOS, revealed their results in a paper published online by the journal Nature and presented here at the winter meeting of the American Astronomical Society.

Just collecting the data required nearly 1,000 hours of observations using the Hubble Space Telescope's best camera, representing 10 percent of the past two years' available observing time.

"It's the largest project that's ever been done by the space telescope," Nick Scoville of the California Institute of Technology, principal investigator for the international research team, told journalists. Additional observations were made by the European Space Agency's XMM-Newton X-ray space telescope, NASA's Spitzer Space Telescope and ground-based telescopes in Hawaii, Chile and New Mexico — involving more than 80 scientists in all.

Locating the missing mass
The dark matter study was aimed at shining new light on one of the deepest mysteries facing astronomers today: Exactly where is the universe's missing mass?

Decades' worth of observations have found that all the matter we can see in surrounding galaxies doesn't account for the gravitational effects of those galaxies. In fact, there appears to be six times more dark matter out there than the ordinary matter we can see.

NASA / ESA / Caltech This photo combines three views of a patch of sky. Ordinary matter (in red) is detected mainly by the XMM-Newton telescope. Hubble charted the distribution of dark matter (in blue) and the distribution of visible stars and galaxies (in black and white).

Is dark matter distributed in the same way as ordinary matter? Computer simulations and studies of individual galaxies, including our own Milky Way, have indicated that it is — and over the past few years, astronomers have been fleshing out their large-scale map of the universe's dark matter. Those maps show galaxies as knots of light caught up within in a "cosmic web" of unseen matter.

The findings announced Sunday provided a "first glimpse of the cosmic web" in true-to-life, three-dimensional detail, said Caltech's Richard Ellis, another member of the COSMOS team.

The lead author of the Nature paper, Caltech astronomer Richard Massey, said the COSMOS study provides the best confirmation that dark matter determines "the underlying structure of space." Galaxies as well as primordial globs of gas and dust form "within this dark-matter scaffolding," he said.

Massey said the findings were consistent with the current mainstream view among astronomers — in which ordinary matter accounts for just 4 percent of the universe, dark matter accounts for another 23 percent, and a mysterious repulsive force known as dark energy takes in the other 73 percent.

From smooth to clumpy
The findings shore up another mainstream theory as well: the idea that the universe started out with a smooth distribution of matter, both ordinary and dark, and became increasingly clumpy as billons of years rolled by.

That view was borne out by the COSMOS results. Astronomers looked farther back in their 3-D map — back as far as 6.5 billion years, when the universe was half its current age — and found that the dark matter was much more evenly distributed then.

Ellis said "it's very reassuring that we see the growth of the dark matter" into a clumpy web structure over time, because it bears out current prevailing theory.

Jason Rhodes of NASA's Jet Propulsion Laboratory, yet another COSMOS team member, said the 3-D map "showed the evolution of the dark matter distribution" over time, and may provide fresh hints for understanding the cosmic tug of war between gravity's attraction and dark energy's repulsion. The clumpiness of matter in the present-day universe may well be the result of that tug of war.

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