"Galaxies rarely collide, but the universe is vast, there are many billions of galaxies, and they have had plenty of time to drift into one another.
During a galactic meetup, theorists say, the supermassive black holes at the cores of the two galaxies first will do a gravitational dance. They can orbit each other for millions of years, Lam said. This pairing is known as a supermassive black hole binary."

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"Supermassive black hole binaries, slowly and calmly orbiting each other, are the tenors and bass of the cosmic opera," Marka said.