Two colossal black holes appear to be orbiting one another in sort of a cosmic minuet at the center of a faraway galaxy formed when two separate galaxies collided, U.S. astronomers said on Wednesday.

These two so-called supermassive black holes, which are celestial objects with enormous gravitational pull, are locked in orbit about 5 billion light years away from Earth, the scientists said. A light year is about 6 trillion miles (10 trillion km), or the distance light travels in a year.

Data from Apache Point Observatory in New Mexico provided the best evidence to date of two black holes orbiting each other, according to astronomer Todd Boroson of the National Optical Astronomy Observatory in Tucson, Arizona.

This is known as a binary system.

Scientists believe all or most galaxies have supermassive black holes at their center. For example, our Milky Way galaxy has a black hole at its center that is about 3 million times the mass of the sun.

When galaxies collide and merge, as they do relatively often, the black holes at their center may gravitate toward one another because of their great mass, entering into orbit as these two appear to be doing.

Boroson said orbiting black holes eventually may merge into an even larger single black hole.

While scientists think binary black holes may be relatively common, they have been elusive.

Boroson and fellow National Optical Astronomy Observatory astronomer Tod Lauer detected these two by spotting the radiation emitted by objects apparently being sucked into the black holes by their gravitational pull.

"You don't see the black holes. You only see the effects of the black holes," Boroson said in a telephone interview.

The smaller of the black holes is about 20 million times the mass of our sun, while the larger one is a billion times the sun's mass, the scientists wrote in the journal Nature.

"We've discovered that there's a pretty good correlation between the size of a black hole and the size of the galaxy that it's in. This could be a case where a big galaxy ate a smaller galaxy," Boroson said.

These two black holes take about a hundred years to orbit each other and are located about three-tenths of a light year from each other, Boroson said.