August 22, 2006

“Dark matter” is real, scientists say

By Scott Malone

BOSTON (Reuters) - A team of U.S. scientists has found the
first direct evidence of the existence of "dark matter," a
little-understood substance with a huge influence on gravity,
the team's leader said on Tuesday.

Scientists still do not know what exactly dark matter is,
but have theorized it must exist to account for the amount of
gravity needed to hold the universe together.

They estimate that the substance accounts for 80 to 90
percent of the matter in the universe. The more familiar kind
of matter, which can be seen and felt, makes up the rest.

Now researchers led by University of Arizona astronomer
Doug Clowe say they have evidence to back up their theories.

Using orbiting telescopes, the researchers watched two
giant gas clouds in outer space collide over a 100-hour period.
As the clouds clashed, they said, the visible gas particles
slowed, pulling away from the invisible dark matter particles.

The researchers said they could detect the dark matter
particles by their gravitational pull on the surrounding
visible particles.

"This is the first time we've been able to show that (dark
matter) has to be out there, that you can't explain it away,"
Clowe told Reuters. "We haven't actually been able to see the
dark matter particles themselves, but what we have been able to
do is ... image the gravity that they're generating."

Some skeptics have argued that dark matter does not exist.

They assert that scientists err in assuming that gravity
exerts the same pull whether holding a plate on a table or
influencing the travel of stars. Revising the laws of gravity
at the interstellar scale would better explain the universe's
structure, they argue.


The research team also included scientists at the
Harvard-Smithsonian Center for Astrophysics in Cambridge,
Massachusetts, and used telescopes operated by NASA.

Their research is scheduled to be published in an upcoming
issue of The Astrophysical Journal Letters.

Rachel Bean, a professor at Cornell University who
specializes in dark matter and was not involved in the
research, called the results convincing.

"It is certainly the strongest evidence we've seen to date
that actually solves this dark-matter problem," Bean said.

She said the finding should encourage scientists to
concentrate their efforts on determining what dark matter is,
rather than developing revised rules of gravity.

"It's very difficult to explain these observations with
anything other than particle theory," Bean said. "The dark
matter quandary to some extent is helped by these observations,
because it helps target the theorists to try and look at
particle physics, rather than gravity."