Researchers have seen the best evidence yet for a pair of black holes orbiting each other within the same galaxy.
While such "binary systems" have been postulated before, none has ever been conclusively shown to exist.
The new black hole pair is dancing significantly closer than the prior best binary system candidate.
The work, published in the journal Nature, is in line with the theory of the growth of galaxies, each with a black hole at their centre.
The theory has it that as galaxies near one another, their central black holes should orbit each other until merging together.
But evidence for black holes nearing and orbiting has so far been scant.
As matter falls into black holes, it emits light of a characteristic colour that in turn gives information about the direction in which the black hole is moving.
In a binary system, two beams should be emitted, each a slightly different colour.
Todd Boroson and Tod Lauer of the US National Optical Astronomy Observatory analysed some 17,500 spectra from the Sloan Digital Sky Survey, and have now found just such a pair of emissions coming from a distant quasar.
The researchers estimate that the two light sources come from black holes between 20 million and one billion times more massive than our Sun.
The black holes are separated by an estimated distance of less than a third of a light-year - cheek-to-cheek by black hole standards and significantly closer than the postulated binary system spotted by the Chandra X-ray Observatory in 2003.
The pair are estimated to dance around one another every 100 years.
Because they are moving with respect to the Earth as well as to each other, observations of their movement over the next few years should prove beyond question that they are indeed the first partnered pair of black holes.
"Previous work has identified potential examples of black holes on their way to merging, but the case presented by Boroson and Lauer is special because the pairing is tighter and the evidence much stronger," said Jon Miller, an astronomer at the University of Michigan.