What tossed a supermassive black hole around in the center of a distant galaxy?

You'd think that tossing around a supermassive black hole equal to the mass of three million times that of the Sun's would be difficult. But for the galaxy J0437+2456 something did exactly that. The question is, what?

The answer, one way or another: Another supermassive black hole.

We think that every big galaxy has a supermassive black hole in its center, with millions or even billions of times the mass of the Sun. In general, we also expect this monster to sit in the exact center of the gala...