From: Reuters
Published May 20, 2008 03:17 PM

Missing matter found in deep space

By Maggie Fox, Health and Science Editor

WASHINGTON (Reuters) - Astronomers have found some matter that had been missing in deep space and say it is strung along web-like filaments that form the backbone of the universe.

The ethereal strands of hydrogen and oxygen atoms could account for up to half the matter that scientists knew must be there but simply could not see, the researchers reported on Tuesday.

Scientists have long known there is far more matter in the universe than can be accounted for by visible galaxies and stars. Not only is there invisible baryonic matter -- the protons and neutrons that make up atoms -- but there also is an even larger amount of invisible "dark" matter.

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Now about half of the missing baryonic matter has turned up, seen by the orbiting Hubble space telescope and NASA's Far Ultraviolet Spectroscopic Explorer, or FUSE.

"We think we are seeing the strands of a web-like structure that forms the backbone of the universe," said Mike Shull of the University of Colorado, who helped lead the study published in The Astrophysical Journal.

The matter is spread as superheated oxygen and hydrogen in what looked like vast empty spaces between galaxies.

However, observations of a quasar -- a bright object far off in space -- show its light is diffused much as a lighthouse can reflect on a thin fog that was invisible in the dark.

"It is kind of like a spider web. The gravity of the spider web is what produced what we see," Shull said in a telephone interview. "It's very thin. Some of it is very hot gas, almost a million degrees."

This is where the dark matter comes in. The dark matter is heating up the gas, Shull said.

"Dark matter has gravity. It pulls the gas in," Shull said. "This causes what I call sonic booms -- shock waves. This shock heats it to a million degrees. That makes it even harder to see."

The atoms of oxygen are in a stripped-down, ionized form. Five of the eight electrons are gone. It emits an ultraviolet spectrum of light that instruments aboard FUSE and Hubble can spot, Shull said.

These web-like filaments of matter are the structure upon which the galaxies form, he said.

"So when we look at the distribution of galaxies on a very large scale, we see they are not uniform," Shull said. "They spread out in sheets and filaments."

Some faint dwarf galaxies or wisps of matter in these structures could be forming galaxies right now, the researchers said.

Shull and colleagues said these webs of hydrogen and oxygen are too hot to be seen in visible light and too cool to be seen in X-rays.

(Editing by Will Dunham and Xavier Briand)

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