U.S. astronomers say they've discovered the biggest galaxy cluster ever seen, a massive grouping of hundreds of galaxies 7 billion light-years from Earth.

Researchers from the Harvard-Smithsonian Center for Astrophysics found the cluster using the South Pole Telescope, a Harvard release said.

"This galaxy cluster wins the heavyweight title. It's among the most massive clusters ever found at this distance," said Mark Brodwin, a Smithsonian astronomer at the center.

Because it's 7 billion light-years distant, we're seeing it as it was 7 billion years ago when the universe was only half its present age and our solar system didn't exist yet, researchers say.

"This cluster is full of 'old' galaxies, meaning that it had to come together very early in the universe's history -- within the first 2 billion years," Brodwin said.

The Harvard-Smithsonian team said it expects to find many more giant galaxy clusters once the South Pole Telescope survey is completed.

"After many years of effort, these early successes are very exciting. The full SPT survey, to be completed next year, will rewrite the book on the most massive clusters in the early universe," Brodwin said.