At the center of nearly every galaxy in the Universe resides a supermassive black hole. When galaxies collide, their supermassive black holes sink to the center of the newly forming galaxy. There in this nascent galactic nucleus a supermassive black hole binary is formed. Supermassive binaries are the subject of a long-standing mystery in astrophysics: will these monstrous black holes merge and what can that tell us about the extreme environments that shape them? I will describe advances in modelling gas interactions with binaries and discuss implications for supermassive black hole binary populations and their observational signatures. I will also discuss a novel cosmological measurement that could be made only from gravitational waves emitted by the most massive of these binaries.