What's out beyond Neptune?

Past Neptune, the last of the eight planets, you might expect nothing but empty black space stretching on forever. For most of history that's exactly what people assumed. But out there, far from the Sun's warmth, lies a colossal ring of icy worlds, thousands upon thousands of them, drifting in the cold and dark. Pluto is just one member of this hidden frozen swarm. What is this place, and why did we not notice it for so long?