for National Geographic News
Astronomers say they have found a type of matter that cannot be seen, but which is thought to dominate the cosmos, in a place where it was thought not to exist.
The finding gives weight to a common theory of how the universe is pieced together.
The exact nature of the matter, called dark matter, is unknown, but scientists believe it accounts for more than 90 percent of the mass in the universe.
Though invisible, scientists think dark matter exists because stars appear to be accelerated by the gravity of a mass greater than all of the visible stars and dust in space.
But this theory was challenged in 2003 when a team of astronomers reported that certain stars in one type of galaxy actually move slowly, suggesting an absence of dark matter.
The new study, based on computer simulations, explains this seemingly odd behavior.
The explanation fits with the theory that these galaxies, like all galaxies, are embedded in haloes of dark matter, said Avishai Dekel, a physics professor at the Hebrew University of Jerusalem in Israel.
"It seems that we have identified the main solution to the problem," Dekel wrote via e-mail.
Dekel is the lead author of the study appearing in tomorrow's issue of the journal Nature.
The theory that galaxies are embedded in dark matter stems from the observation of stars in spiral galaxies. These galaxies, including the Milky Way, are flattened spiral-shaped disks of stars and gas.
Since most of the visible matter in a spiral galaxy is concentrated at the center, stars there would be expected to move more quickly than stars on the outskirts of the galaxy, according to the laws of gravity.
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