for National Geographic News
A team of researchers has found the first direct proof for the existence of dark matter, the mysterious and almost invisible substance thought to make up almost a quarter of the universe.
Dark matter does not absorb or emit light. So far, astronomers have inferred its presence only indirectly by measuring the effects of its gravity.
But now, by observing a massive collision between two large clusters of galaxies, astronomers have detected what they say could only be the signature of dark matter.
The scientists used optical and x-ray telescopes to measure the location of mass in the collided formation, known as the "bullet cluster" because of its shape.
More than 90 percent of the visible mass in a galaxy cluster is hot gas. The rest is stars located within individual galaxies.
The composite image at left shows that this hot gas (red) was dragged away from the stars and galaxies (blue) during the collision (see bigger photo).
But most of the massand thus matteris located within the galaxies, or the blue areas, scientists say.
In other words, the bulk of visible matter in the clusters has been separated from the majority of masswhich therefore must be dark matter.
"This proves in a simple and direct way that dark matter exists," Maxim Markevitch of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, said in a telephone briefing Monday with reporters.
Scientists calculate that dark matter makes up about 25 percent of the universe.
By contrast, ordinary matterthe stuff that makes up stars, planets, and everything on Earthmakes up no more than about 5 percent of the universe.
The other 70 percent of the universe, scientists believe, is made of dark energy, an even more elusive force that is pushing the universe apart at an ever increasing rate.
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