Supermassive black holes lie at the center of most galaxies, but 750 million light-years away, a huge galaxy known as 0402+379 boasts two of them – and one appears to be orbiting the other. The discovery is the result of 12 years of observation, and while it's long been suspected, this marks the first time the phenomenon has been directly seen.

Massive and elliptical, 0402+379 is an unusual galaxy. In 2006 it was discovered to be home to a supermassive black hole binary, and it's been under observation by radio telescopes in the Very Long Baseline Array (VLBA) ever since. Between them, the black holes share a mass of about 15 billion times that of the Sun, ranking them among the largest ever found, and with just seven parsecs separating them, they're also the closest known pair.

Twelve years of VLBA observations have shown astronomers that one of the black holes is orbiting the other, albeit extremely slowly. The team calculated that a single orbit may take the black hole some 24,000 to 30,000 years to complete, which makes its motion very difficult to detect.

"If you imagine a snail on the recently discovered Earth-like planet orbiting Proxima Centauri – a bit over four light years away – moving at one centimeter a second, that's the angular motion we're resolving here," says Roger W. Romani, co-author of the paper.

These black hole binaries are thought to be caused by cosmic collisions between galaxies. Long ago, 0402+379 may have gotten its bloated shape and double-black-hole-center from merging galaxies, and it could teach scientists a lot about the lifespan of galaxies – especially since the same fate awaits the Milky Way, with the Andromeda galaxy speeding towards us right now.

"For a long time, we've been looking into space to try and find a pair of these supermassive black holes orbiting as a result of two galaxies merging," says Greg Taylor, lead researcher on the study. "Even though we've theorized that this should be happening, nobody had ever seen it until now."

Ultimately, the black holes involved in these mergers are expected to blend together as well, creating gravitational waves. Early last year, LIGO detected these ripples in space-time caused by merging black holes, but those two only had a mass of about 30 times the Sun. If 0402+379 ever collide, the resulting gravitational radiation burst would be larger by a factor of a billion.

But that's a pretty big "if", according to the researchers. Given the extremely slow orbit, the universe will probably end before the two black holes ever become one – making them the most tantalizing "will they?/won't they?" couple in the universe we know of.

The research was published in The Astrophysical Journal.