system
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human
Why do supermassive black holes merge in galactic collisions?

Since interstellar space is mostly empty, when the Milky Way and Andromeda galaxies merge, stars are unlikely to collide.

Even things as wide as supermassive black holes will likely miss each other.

How do they end up merging?

* It's highly unlikely they'll be aimed right at each other in the merger.
* They'll almost certainly have mutual angular momentum leading them to orbit each other in big galactic-scale orbits
* But they won't be close enough for gravitational waves to radiate away more than a negligible amount of angular momentum and energy, as would be the case in the final stages of black hole or neutron star mergers

Is it all the virial theorem, with the scattering of stars disbursing energy and anguish momentum?
gpt
>Is it all the virial theorem, with the scattering of stars disbursing energy and anguish momentum?

Pretty much - it's called "dynamical friction". If you have a compact massive object moving through a medium made up of lots of little objects, the gravity of massive object will tend to concentrate the small objects behind it in its orbit, and that creates an ongoing drag-like force. This causes the supermassive black holes to slow down and spiral towards the centre.

However, once they're a few light years apart, there's just not enough stars left for this to be effective, and they're still too far apart for gravitational waves to matter. This is known as the "final parsec problem". It's not actually clear how or even if supermassive black holes routinely get close enough for gravitational waves to cause them to lose loads of energy and merge. There may be lots of galaxies with fairly tight binary supermassive black holes in the centre, for instance.