You could say that the odds of seeing it were astronomical.

Yet there it was, 10 billion light-years from Earth, the most massive galaxy cluster ever seen at such a distance - with a gravity field so strong that it distorted the light of the galaxy behind it in a rare display called gravitational lensing.

"When I first saw it, I kept staring at it, thinking it would go away," said University of Florida astronomer Anthony Gonzalez, lead author of the study announcing the discovery. "The galaxy behind the cluster is a typical run-of-the-mill galaxy with a lot of young stars, but the galaxy cluster in front of it is a whopper for that range. However, it's really the way that the two systems are lined up that makes the occurrence truly remarkable."

Gravitational lensing, or bending of light from the distant galaxy, has never been observed behind a cluster at this range.

The team reports its findings in the July 10 issue of The Astrophysical Journal.

The cluster's mass is noteworthy because its distance from the telescope, 10 billion light-years, means that it was already in existence when the universe was only one-quarter of its present age. Currently accepted models for how the universe evolved suggest that relatively few of these clusters were around when the universe was young.

Just finding the cluster itself might be considered lucky considering the team was exploring a mere 9-square degree patch of sky.

"If you extend your arm in front of you and hold up your index finger," Gonzalez said. "Your finger covers approximately 1-square degree of sky." So finding a massive cluster at that range that is also gravitationally lensing is a real long shot, even if you were looking at the whole sky, he said.

Researchers initially found the cluster using NASA's Spitzer Space Telescope. But they first saw evidence of the gravitational lensing in 2010 images taken by the Hubble Space Telescope. They used data from the Combined Array for Research in Millimeter-wave Astronomy, or CARMA, radio telescope and NASA's Chandra X-ray Observatory to independently verify its mass and distance.

Gonzalez said that finding a cluster of that magnitude at that distance in such a small field of observation could mean that scientists have underestimated how many massive clusters were in existence 10 billion years ago. In other words, perhaps the odds of spotting the cluster were not as great as the models predict.

"We just don't know," he said. "We need to find more clusters at this range so that we can get more data. So far we only have one example to study."

And you might say that they are lucky to have that.