Just like humans, black holes come in singles and pairs. And when a third black hole joins an existing pair, the three-body dynamics is often chaotic. Again, just as in human relationships.
Most galaxies host a supermassive black hole at their center. This is the case for our Milky-Way galaxy and its nearest neighbor, the Andromeda galaxy. These two galaxies are on a collision course. In a few billion years, before the Sun will die, the two galaxies will merge. In 2008, I wrote with my former postdoc T.J. Cox the first paper forecasting this merger to a bigger galaxy which I labeled “Milkomeda”.