Astronomers have found a piece of the universe's puzzle that's been missing for awhile: a type of extremely hot, dense matter that is all but invisible to us.
Engaging in something like cosmic accounting, astronomers have tried to balance the scant amount of matter that has been directly observed with the vast amount that remains unobserved directly. The latter constitutes about 90 percent of the universe's matter.
Galaxies, the stars within them, the planet we live on and the chairs we sit on are made up of normal matter — the protons, electrons and neutrons that are collectively called baryons. Baryonic matter can be seen and directly observed, but it makes up only about 4 percent of the universe.
The rest of the universe is split up between dark matter (about 21 percent), a mysterious type of matter that has yet to be identified but that is thought to have played a critical role in the development of the first galaxies that formed after the Big Bang, and the even more mysterious dark energy (about 75 percent of the universe), which causes the accelerated expansion of the universe.
Dark matter remains a total mystery. But the new study squares the balance sheet a bit in regards to baryonic matter.
Previously, only about half of the baryonic matter in the universe was accounted for by the known gas, stars and galaxies. A team of astrophysicists has now found evidence of part of the missing half in a bridge-like filament connecting two clusters of galaxies. The finding is detailed in the May 2008 issue of the journal Astronomy and Astrophysics Letters.
Along with dark matter, the missing baryonic matter is thought to form an enormous spider web of tendrils that connect galaxy clusters, which sit on threads and knots in the web.
The missing part of this matter was thought to be a hot, ultra-thin gas haze of very low density between larger structures. Its hellacious temperature means that it only emits far-ultraviolet and X-ray radiation.
Some of this missing matter was found by the astrophysicists, who hail from the Max Planck Institute for extraterrestrial Physics (MPE) and the European Southern Observatory in Germany, as well as the SRON Netherlands Institute for Space Research. The team used the XMM-Newton, an X-ray space observatory, to observe a filament connecting two clusters of galaxies, Abell 222 and Abell 223.
"So far we could only see the clusters, the dense knots of the web. Now we are starting to see the connecting wires of the immense cosmic spider web," said MPE study team member Aurora Simionescu of the discovery of this missing baryonic matter.
A similar baryonic haze, 150 times hotter than the sun's surface, was indirectly detected surrounding the Milky Way and connecting about three dozen other galaxies known collectively as the Local Group in 2003 by astronomers at Harvard and Ohio State Universities.
It is thought that these hot intergalactic hazes were created from material that did not fall into galaxies when they first formed more than 13 billion years ago. Finding and analyzing these filaments could help astronomers better understand what happened after the Big Bang and what forces are dominating the universe today.