I've read multiple physicists talk about stuff (e.g. galaxies, black holes, etc.) "existing" outside the observable universe, but it's unclear to me in what sense these things "exist". By definition things outside the observable universe are spacelike separated, hence casually disconnected, from us. And since two spacelike separated events can always be made simultaneous by taking the right reference frame, we can always pick a reference frame in which any given event outside the observable universe "hasn't happened yet". It seems to me the more accurate thing to say is that the observable universe is "growing" and that "outside the observable universe" is a collection of events that "haven't happened yet". What do physicists mean when they talk about things existing outside the observable universe?