One of the great, unsolved mysteries of 21st-century science is the existence of the missing matter of the universe. Using observations with NASA's Chandra X-ray Observatory and ESA's XMM-Newton, astronomers have announced a robust detection of a vast reservoir of intergalactic gas about 400 million light years from Earth. This discovery is the strongest evidence yet that the "missing matter" in the nearby Universe is located in an enormous web of hot, diffuse gas.
This missing matter — which is different from dark matter -- is composed of baryons, the particles, such as protons and electrons, that are found on the Earth, in stars, gas, galaxies, and so on. A variety of measurements of distant gas clouds and galaxies have provided a good estimate of the amount of this "normal matter" present when the universe was only a few billion years old. However, an inventory of the much older, nearby universe has turned up only about half as much normal matter, an embarrassingly large shortfall.
The mystery then is where does this missing matter reside in the nearby Universe? This latest work supports predictions that it is mostly found in a web of hot, diffuse gas known as the Warm-Hot Intergalactic Medium (WHIM).
Scientists think the WHIM is material left over after the formation of galaxies, which was later enriched by elements blown out of galaxies.
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http://www.dailygalaxy.com/my_weblog/2010/05/chandra-space-observatory-locates-dark-missing-matter-of-the-universe.html Using observations and simulations, scientists calculate the WHIM has a density equivalent to only 6 protons per cubic meter. For comparison, the interstellar medium -- the very diffuse gas in between stars in our galaxy -- typically has about a million hydrogen atoms per cubic meter.
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