Unknown: suppose I say there's a central singularity in each galaxy, so there's one singularity to begin with, and two at the end, but there's no exact moment when one singularity becomes two. That's what you're doing when you say there's an observer in each world, and then vague out on the concept of "world".

Does the truth of a sentence exist? A proper discussion of that might explode the boundaries of this blog again. But I'll just say that I had ostensive definitions in mind, when I said that the referent of a concept may be known even when its nature is not. If I point to a light in the sky and say, "that's Venus", you know what "Venus" refers to, even though you may not know much about it. And both "existence" and "truth" similarly admit of "definition"-by-example, i.e. by exhibition of an instance.