Astronomers believe that supermassive black holes, billions of times heavier than the Sun, lie at the centre of every galaxy. Moreover, it is thought that some galaxies might even contain two supermassive black holes as a result of two smaller galaxies colliding and merging over the course of galactic evolution. However, today all evidence for supermassive black hole binaries has been indirect.

Although black holes do not emit light, the gravitational field of a supermassive black hole is strong enough to alter the motion of surrounding objects in a characteristic way. The Japanese team observed the orbital motion of 3C 66B, which lies close to a suspected supermassive black hole pair, between March 2001 and June 2002 with the very-long-baseline array of the National Radio Astronomy Observatory. They found that the core of the radio galaxy traced a well-defined ellipse with an estimated orbital period of about 1.05 years.

Sudou and co-workers also calculate that the black hole pair is more than a billion times heavier than the Sun. This value is consistent with 3C 66B being a giant elliptical galaxy and suggests that it was formed by two galaxies merging.