Supermassive black holes are generally at the center of galaxies, like a tiny yolk in an egg. That's not a hard rule. For instance, they can theoretically be ejected and make a mess if a few galaxies collide. But yes, it's harder to detect far away objects when the bulk of our milky way is in the way, making things milky. I suspect there are a lot of black holes back there that we just can't see.