It might seem hard enough to swallow that most of the universe's matter is a kind we can't see and don't understand. But scientists have to go a step further and suggest that some of this "dark matter" is hot and some is cold.
Now a new study shows that about 80 percent of dark matter is the cold variety.
Nobody knows what dark matter is. But astronomers know it exists because without it, galaxies could not look like they do -- stars simply wouldn't be held into the orbital formations that are seen. Theorists imagine dark matter being subatomic particles of some sort that interact with each other and "normal" matter only through gravity.
Cold dark matter is presumed to exist based on a theory holding that that dark matter particles in the early universe were moving slowly when galaxies and galaxy clusters began to form.
In the new study, astronomers used NASA's Chandra X-ray Observatory to make the most detailed probe yet of the distribution of dark matter in a massive cluster of galaxies.
Chandra observed Abell 2029, a galaxy cluster located about a billion light-years from Earth. Thousands of galaxies are wrapped up in a big cloud of hot gas along with an amount of dark matter equal to more than a hundred trillion Suns. At the center of the cluster is an enormous, elliptically shaped galaxy thought to have been formed from the mergers of many smaller galaxies.
The X-ray data show that the density of dark matter increases smoothly all the way into the center of the cluster. This discovery agrees with predictions of cold dark matter models and is contrary to other models that predict a leveling off of the amount of dark matter in the center of the cluster.
"We still have very little idea as to the exact nature of these particles, but our results show that they must behave like cold dark matter," said Aaron Lewis of the University of California, Irvine, lead author of a paper describing the results in a recent issue of the Astrophysical Journal.
The observations showed the distribution of X-rays from the hot gas, which is held in the cluster primarily by the gravity of the dark matter, Lewis and colleagues explained in a statement Wednesday, so the distribution of the hot gas reveals the distribution of dark matter.
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