Dark Matter Proof Found, Scientists Say

John Roach
for National Geographic News
August 22, 2006
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 mass—and thus matter—is 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 mass—which 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 matter—the stuff that makes up stars, planets, and everything on Earth—makes 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.

Weighed Down

Scientists have known for more than 70 years that galaxy clusters have too much gravity to be explained by the amount of visible matter alone, according to Douglas Clowe, an astronomer at the University of Arizona in Tucson.

Something must be providing extra gravity, or the galaxies would quickly fly apart.

"This extra gravity has two possible explanations," Clowe said during the briefing.

The traditional explanation is that a form of invisible matter—dark matter—makes up the majority of mass in a galaxy cluster.

An alternate theory is that gravity behaves differently over large distances—such as on the scale of galaxy clusters, which are typically a few million light years in size—than it does on Earth and in our solar system.

Dark matter remained the dominant theory until recently, because astronomers have not had complete theories of alternate gravity to test, Clowe said.

"As a result, astronomers have been in the slightly embarrassing position of having to explain their observations using something we didn't know actually existed," he said.

Crash Test

The collision of the galaxy clusters, which the Harvard-Smithsonian Center's Markevitch says produced enough energy "to completely evaporate and pulverize the planet Earth ten trillion, trillion times," provided a chance to test for the existence of dark matter.

Most of the visible mass in clusters of galaxies is in the form of hot gas or stars. But the mass of hot gas found between individual galaxies is far greater than the mass of stars found within those galaxies, the researchers say.

The galaxy cluster collision—which happened about a hundred million years ago—pulled the gas and the galaxies apart.

During the collision, the hot gas was slowed by a drag force similar to air resistance and separated from the rest of the cluster, the team explains.

But scientists knew from earlier observations that dark matter—if it exists—will not be slowed by such a drag force, because it does not interact with itself or the gas except through gravity.

(Related story: "Dark Matter Properties 'Measured' For First Time, Study Says" [February 13, 2006].)

The team used the Chandra x-ray telescope to image the hot gas, which makes up about 90 percent of the normal matter in the collided galaxy clusters.

The new formation is known as the bullet cluster, because the gas forms a bullet shape in the smaller colliding cluster.

Meanwhile, the Hubble Space Telescope, European Southern Observatory's Very Large Telescope, and Magellan optical telescopes were used to determine the location of mass in the cluster.

The mass was determined using a phenomenon called gravitational lensing, which occurs when, as predicted by Einstein's theory of general relativity, the path of light is distorted by gravity. The amount of mass can be calculated from the amount of distortion.

The composite image of the galaxy clusters shows that the majority of the mass is centered over the galaxies (blue), away from most of the ordinary matter—the hot gas (red) (see bigger photo).

"If there was no dark matter, then the blue and the red clouds would be on top of each other, as the hot gas has most of the visible mass which we see in the system," the University of Arizona's Clowe said.

"What you can see clearly, though, is that the gravitational lensing signal is located near the galaxies and is not located near the gas clouds. What this tells us is there has to be dark matter present to explain this," he continued.

Clowe is the lead author of a paper on this research accepted for publication in an upcoming issue of the Astrophysical Journal Letters.

Astronomy Redefined

Sean Carroll is an assistant professor of physics at the University of Chicago in Illinois who was not involved with the study.

At the briefing, he said the study marks "a change in what cosmology is all about."

In 20th-century cosmology, scientists measured how big the universe is and what it's made of.

Now that scientists know the answer to those questions—including the existence of dark matter—Carroll said they can turn to the next question: "Why?"

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