And scientists think they know how to find out for sure.
We already know our galaxy, the Milky Way, as home to Sagittarius A*, a black hole with a mass about 4 million times that of the Sun.
But an international team of astrophysicists suspects that Sgr A* might have a smaller companion black hole. Now, they think they've figured out how to determine if they're right.
On Thursday, UCLA astrophysicistBased on their observations of star S0-2, which orbits Sgr A* every 16 years, the researchers know that the Milky Way's supermassive black hole couldn't have a companion with a mass greater than 100,000 times that of our Sun. But they aren't ruling out a smaller "hidden friend."
Naoz believes the Laser Interferometer Space Antenna, a mission expected to launch in the 2030s, might allow scientists to determine whether or not Sgr A* indeed has a companion by detecting its gravitational waves.
If the Milky Way is home to two black holes, that would imply it once merged with another galaxy, Naoz wrote — meaning Sgr A*'s "hidden friend" could tell us something new about our galaxy both today, and in the distant past.
READ MORE: Supermassive black hole at the center of our galaxy may have a friend [The Conversation]
More on black holes: NASA’s New Black Hole Simulation Will Completely Melt Your Brain
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