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, according to Reuters. 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."
