I'm a total newbie at this topic, as I'm still doing my GCSEs (10th grade), but hasn't it been said that previously, 2 galaxies have collided, and together have joined to form 1 galaxy. From my knowledge, there are supermassive black holes (which are millions to billions times the mass of the sun) in the centre of galaxies, as there is one in the Milky Way. Somehow, I suppose, these black holes have joined to make one (NoteL This may be completely wrong, and I may have been misguided, because I also thought, as the above post states, that those two black holes were the closest apart!). One may have "consumed" the other, I am not sure. But if they are exactly the same size, maybe they would do orbit one another, like planets do. But I suppose that it would not be a normal circular motion. One would not be orbiting the other, they would be orbiting each other simultaneously. I have no idea what I'm talking about right now, but I think I'll stop there, before I confuse myself and others anymore!