The new picture shows stark dots representing galaxies that emit X-rays, collectively revealing the large-scale structure of the universe about five billion years ago. The universe is thought to be 13.7 billion years old now, and no one knows how old it will ultimately become.

The image was then blurred, on purpose, to "to allow better view of the structures outlined," astronomers said this week at a meeting of the High Energy Astrophysics Division of the American Astronomical Society at Mt. Tremblant, Quebec.

"We are seeing the universe during its formative years," said Richard Mushotzky, a researcher at NASA's Goddard Space Flight Center in Greenbelt, Md. who led the observation. "This is billions of years after galaxies were born, during a period when the universe began to take on the trappings of an adult."

The observation traced a patch of sky known as the Lockman Hole in the constellation Ursa Major, which contains the Big Dipper. Chandra saw a rich density of active galaxies, seven times denser than what has been detected in previous optical and radio surveys at similar distances. This provides the clearest picture yet at the large-scale structure of the universe at such distances and age, Mushotzky said.

The universe's large scale structure -- galaxies, galaxy clusters and voids of seemingly empty space -- take on the appearance of a web when viewed in distinct patches. Galaxies and intergalactic gas are strung like pearls on unseen filaments of mysterious dark matter, which comprises more than 85 percent of all matter.

Galaxies are attracted to dark matter's gravitational potential, just as they are attracted to regular matter.

Astronomers have never seen dark matter and do not know what it is. Chandra's observation of distant galaxies in the Lockman Hole, spread out over several billion light years from Earth, essentially maps the distribution of dark matter, however. This provides clues to how the universe grew.

"Piece by piece, we are assembling a photo album of the universe through the ages," said Yuxuan Yang, a doctorate candidate at the University of Maryland, College Park, who conducted the analysis.

The galaxies that the team saw with Chandra were either dim or altogether undetectable with optical and radio telescopes. This may be because they are enshrouded in dust and gas, which blocks radio waves and optical light. X-rays, a higher-energy form of light, can penetrate this shroud.

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