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Scientists say they have proof of dark matter

Two galaxies' collision studied

WASHINGTON -- 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 exists.

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

While the scientists are not sure exactly what dark matter is, since they have yet to identify it in a laboratory, they said 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 the workings of gravity. 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 by using data from NASA's orbiting Chandra X-ray Observatory, and involved information from what researchers called the most massive releases of detected energy in the universe since the Big Bang.

Scientists said a collision between the enormous ``bullet" cluster of galaxies more than 3 billion light years away and another smaller galaxy cluster proved the existence of dark matter by, in effect, stripping 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."

The Chandra X-Ray Center, linked to the Smithsonian Astrophysical Observatory, is in Cambridge.

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 counter theory has grown contending that the laws of gravity established by Newton and Einstein need modification.

This group says 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 undetectable dark matter does.

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 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 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, 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," said Carroll. ``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."

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