Scientists studying data from the 
Chandra X-ray telescope have
found strong evidence for the missing half of ordinary matter
in the universe. Two immense intergalactic clouds confirm the
existence of a cosmic web of hot gas, predicted by computer
simulations, containing baryons, the material making
up visible objects in the Universe.
"An inventory of all the baryons in stars and gas inside and outside
of galaxies accounts for just over half the baryons that
existed shortly after the Big Bang," Fabrizio Nicastro of the
Harvard-Smithsonian Center for Astrophysics.  "Now we have found the
likely hiding place of the missing baryons."  Nicastro is lead author
of a paper in the February 3, 2005 issue of 
Nature describing the recent research.
Various measurements give a good estimate of the mass-density of
baryons - the neutrons and protons
that make up the nuclei of atoms and ions - in the
Universe 10 billion years ago. However, sometime
during the last 10 billion years a large fraction of the baryons,
commonly referred to as "ordinary matter" to distinguish them
from dark matter and dark
energy, have gone missing.
Using the computer predictions, Nicastro and colleagues
searched specifically for the diffuse system of gas clouds
galaxies and galaxy clusters form within.  Evidence of the
clouds had been previously detected around our own galaxy and in
the Local Group, but the temperature range (a few
hundred thousand to a million degrees Celsius) and low density
have prevented detection outside the immediate neighborhood of
galaxies and the reliable estimation of baryon density for the
entire universe.
The clouds were detected using Chandra observations when the
galaxy Mkn 421 brightened in X-ray
emissions.  The X-ray spectral data showed two hot gas clouds,
150 million and 370 million light years distant, absorbing the
X-rays from Mkn 421. The data shows the presence of carbon,
nitrogen, oxygen, and neon present in the clouds. Combined with
other data the research team determined the thickness and mass of
the clouds.
Assuming that the size and distribution of the clouds are
representative, Nicastro and colleagues made the first reliable
estimate of average density of baryons in such clouds throughout
the Universe, finding it consistent with the mass density of the
missing baryons.
Mkn 421 was observed three times with Chandra's Low-Energy
Transmission Grating (LETG), twice in conjunction with the High
Resolution Camera (May 2000 and July 2003) and once with the
Advanced CCD Imaging Spectrometer (October 2002). The distance to
Mkn 421 is 400 million light years. 

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Chandra telescope finds missing baryonic matter