Scientists Offer Proof of 'Dark Matter'

This composite image depicts the
This composite image depicts the "bullet cluster" formed by galactic collision, with hot gas in a bullet shape shown in red and areas of dense matter in blue. Most of the matter in blue area is "dark," according to the astronomers' analysis. (NASA/AFP/Getty Images)
By Marc Kaufman
Washington Post Staff Writer
Tuesday, August 22, 2006

For decades, many scientists have theorized that the universe is made up of nearly undetectable mysterious substances called dark matter and dark energy. But until yesterday there was no proof that the subatomic matter actually exists.

After studying data from a long-ago collision of two giant clusters of galaxies, researchers now say they are certain dark matter does exist and plays a central role in creating and defining gravity throughout the universe.

While the scientists are still not sure exactly what dark matter is, since they have yet to identify it in a laboratory, they said that the workings of the universe cannot be explained without it.

The finding will have potentially great impact on an active debate among physicists and cosmologists about not only dark matter but also the workings of gravity that it helps explain. Indeed, the theory of dark matter evolved largely to explain the finding several decades ago that there was not enough visible matter in the universe to produce and account for the gravity needed to keep galaxies from flying apart.

"A universe that's dominated by dark stuff seems preposterous, so we wanted to test whether there were any basic flaws in our thinking," said Doug Clowe of the University of Arizona in Tucson, leader of the NASA-Harvard University study. "These results are direct proof that dark matter exists."

The breakthrough came using data from NASA's orbiting Chandra X-Ray Observatory and involved information from what researchers called the most massive release of detected energy in the universe since the big bang.

Scientists said that the "bullet cluster," formed by a collision between an enormous cluster of galaxies more than 3 billion light-years away and a smaller galaxy cluster, demonstrated the existence of dark matter. In effect, the collision stripped the dark matter away from visible matter. Once stripped, dark matter was clearly identified by the strong gravitational pull that it exerted.

"We now have direct evidence" of dark matter, said Sean Carroll, a cosmologist in the physics department of the University of Chicago, who did not participate in the study. "There is no way to explain the observations without dark matter."

While the theoretical existence of dark matter has been broadly embraced for years -- and has now been further endorsed by some of the most prominent researchers and institutions in the field -- a strong countertheory has also grown, contending that the laws of gravity established by Newton and Einstein need modification. The group supporting this theory believes that a relatively limited tweaking of those laws, especially as they pertain to the massive nature of faraway galaxies, could explain the missing gravity better than could undetectable dark matter.

Stacy McGaugh, an astrophysicist at the University of Maryland, has been one of the dark-matter skeptics, and he said yesterday that he remained unconvinced.

"I've been aware of this result some time, and I agree that it is interesting and may make more sense in terms of dark matter than alternative gravity," he said. "However, it is premature to say so."

He said that a definitive detection of dark-matter particles would mean "grabbing them in the laboratory, not just inferring that their effects can be the only possible explanation for an observation before the alternatives have actually been checked."

The NASA-affiliated team that announced its findings yesterday said that the next step in trying to understand dark matter (and related dark energy) is, in fact, to identify it in a laboratory. That task has proved difficult so far, they said, because dark matter leaves no detectable traces, except to create a gravitational pull.

"This finding doesn't tell us where dark matter comes from," Carroll said. "It tells us that dark matter exists, but it doesn't say what it is, or why there's so much of it. The real adventure is ahead of us."

The researchers said yesterday that visible and detectible matter -- the atoms in everything from gases to elephants and stars -- makes up only 5 percent of the matter in the universe. Another estimated 20 percent is subatomic dark matter, which has no discernible qualities except the ability to create gravitational fields and pass through any object without leaving a trace. The rest, they said, is the even more mysterious dark energy, which fills empty space with a force that appears to negate gravity and push the universe to expand ever faster.

According to team member Maxim Markevitch of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., their discovery was made only because the Chandra observatory was able to clearly track the effects of the collision between the two galaxy clusters. Monitors on Chandra, which has an elliptical orbit that sometimes carries it one-third of the distance to the moon, were able to detect and describe an unusual process in which the super-hot gases of the galaxy clusters separated from the remaining stars.

The super-hot gases have qualities that typically would have become the seat of any new gravitational fields, cosmologists say, but instead they went with the stars. That could happen, Markevitch said, only if dark matter separated from the gases and collected with the stars.

The team's paper will be published in the Astrophysical Journal Letters.

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