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.

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