![July 2009 black holes](https://www.astronomy.com/wp-content/uploads/sites/2/2023/02/july_2009_black_holes.jpg)
Two black holes merge in this computer-generated image based on a NASA simulation. As the black holes approach each other, their violent interaction generates ripples in the fabric of space-time.
Chris Henze (NASA)
When galaxies merge, their central black holes combine to form a single, more massive black hole.