Dark matter could play ‘matchmaker’ for supermassive black holes. Dark matter, the most mysterious “stuff” in the universe, may be a sort of cosmic matchmaker, allowing supermassive black holes to overcome a final hurdle before colliding and merging into one.

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*Alonso-Álvarez explained that, when galaxies first merge, their respective supermassive black holes are far apart. Eventually, they begin their respective journeys toward the center of those merging galaxies, which will become the site of their own merger and the heart of a newly created conglomerate galaxy.*
*This journey is facilitated by the black holes’ loss of energy, and it happens in three distinct stages.*
*During “stage one,” the black holes sweep past a multitude of stars and engage in gravitational interactions with those stars. Due to those interactions, the black holes pass along velocity and slow down. This loss of velocity allows the concentration of mass at the center of the merger to gravitationally influence the black holes to head (somewhat leisurely) toward one another.*
*That loss of energy continues to be important after the final 3.3 light-years are crossed and the supermassive black holes form a binary system, which is stage three of this process. At this point, the voids swirl around each other, generating gravitational waves. As these waves ripple away from the binary system, they carry with them angular momentum, thus losing rotational kinetic energy.*
*This very efficient loss of energy causes the two black holes to draw closer together, with their merger becoming unstoppable. The tighter their orbits become, the faster gravitational waves are radiated away. Eventually, as the supermassive black holes get closer and closer together, their gravitational influence takes over, and they collide.*