When two galaxies collide, their central black holes begin a long dance of approach. Until they merge into one, we can see a pair of quasars — bright beacons that ignite as the black holes consume gas and dust. Such pairs in the sky are like two lighthouses standing close together, each shining from its own harbor.

Astronomers combed through data from large sky surveys and found 17 such pairs. Selecting them was no easy task: first, they filtered out any objects that shifted even slightly in images taken at different times. Real quasars are so distant that they appear motionless, like distant mountains on the horizon. Then, for each candidate, they took a spectrum, splitting the light into colors like a rainbow, to confirm that both sources are indeed at the same distance from us.

Among the 143 random coincidences, where quasars merely project close together on the sky, four pairs were especially close — less than 30 kiloparsecs apart. That's still far: light from one to the other would take a hundred thousand years. But for astronomers, this is a chance to probe the gas halo of the foreground galaxy using absorption lines in the spectrum of the background quasar, as if a distant flashlight illuminated the fog along its path.

The 17 confirmed pairs are not just points on a map. Each pair is a future binary system of supermassive black holes that, in billions of years, will merge and produce gravitational waves. For now, they flicker in the depths of space, reminding us that even the largest structures in the universe were once just two neighbors who happened to meet at the edge.