A surprisingly sturdy little galaxy is barging right into the Earth's own Milky Way but it will probably be sucked in rather than cause any damage, astronomers said Friday.

And no one on Earth needs to worry - it probably won't happen for another 3 billion to 5 billion years, and in any case would have little effect on this planet.The little galaxy, known as the Sagittarius dwarf spheroidal galaxy, could help astronomers figure out the secret of "dark matter," which measurements show must make up about 90 percent of the universe but which has yet to be seen.

"Is there dark matter in this galaxy?" Rosemary Wyse, an astrophysicist at Johns Hopkins University in Baltimore, asked at a news conference at the American Association for the Advancement of Science annual meeting in Philadelphia.

"Is it the same as dark matter we believe existed in larger galaxies?"

Wyse worked with a team of astronomers including Rodrigo Ibata from the European Southern Observatory in Chile, and Mike Irwin at the Royal Greenwich Observatory, and Gerard Gilmore of the Institute of Astronomy in Britain. These three originally saw the Sagittarius dwarf galaxy in 1994.

They and Wyse have made new observations that show the galaxy must orbit our Milky Way every billion years. Gravity from the much larger and massive Milky Way should have started pulling it apart, but it hasn't.

Wyse says this must come down to dark matter - matter that cannot be seen by conventional means but which makes its presence known by its effects on gravity, and on galaxies like the Sagittarius dwarf.

"It's just got a lot of dark matter, so it's able to hold on to its stars," Wyse said.

Having this dwarf galaxy nearby might let astronomers have a close-up peek at what dark matter looks like.

Dark matter is key to the future of the universe. How much matter there is will determine if the Big Bang that started it all will end in a Big Crunch or a never-ending expansion.

Wyse said dark matter might include dim stars, but there had to be more to it than that. "One of the interesting things is that there might be more than one kind of dark matter," she said.

Right now it can be seen only by its gravitational effects - it pulls on light from far-away galaxies, an effect called gravitational lensing.

Most galaxies, including the Sagittarius dwarf, seem to have a big halo of dark matter that attracts them to one another as they pass by on their cosmic journeys. There is so much it creates a kind of wake as they come together, said Chris Mihos of Case Western Reserve University in Ohio.

Having Sagittarius merging so close by also gives astronomers a chance to see what might happen on a bigger scale, when the Milky Way's next-door neighbor, the Andromeda galaxy, comes crashing in.

The Andromeda galaxy is 2 million light years away, a light year being the distance light travels in a year at 186,000 miles per second, so it will not hit the Milky Way for billions of years.

Mihos said there was no worry to any Earthlings that might have survived that long.

"If you shrunk the Sun down to the size of a marble, the next star would be somewhere out in the Caribbean," he said.

"So we don't have to worry about another star smacking into our star."