Not all black holes stay in galaxies; some can be ejected by gravitational interactions or collisions and roam intergalactic space. These rogue black holes can weigh millions of suns and move at hundreds of kilometers per second.
It matters because wandering black holes affect galaxy evolution and dynamics.
It also raises questions about detecting isolated massive objects in space.
Some black holes drift alone through space, far from any galaxy.
[NASA, nasa.gov]