Although they are called "clusters," world clusters actually have no boundaries. They are domains composed of infinite worlds. Even an expert at the Unity Realm couldn't traverse a single Primordial World Cluster in their entire lifetime. According to statistics from the Three-Life Race, the number of known, inhabited worlds doesn't even amount to a hundred-millionth of a hundred-millionth of the unknown, inhabited worlds.
It's like tossing a single known grain of sand into a vast desert. That one grain represents the known worlds, while the rest of the desert represents the unknown.
