4. "They" Are Not Aliens, But A Future Form Of Humanity
As Cooper explores this bizarre expanse, allowing him to observe Murph as both a child (when she first experiences the "ghost") and as an adult, he comes to realise that, while humans are not currently equipped to deal with time and space in such a nebulous manner, future humanity almost certainly will be able to, a form so far advanced that they need not deal with time and space in the precise terms that modern humans do. Cooper then appreciates that these mysterious future humans have traveled back through time, creating the wormhole to keep humanity alive in the past. Why they couldn't have made the damn wormhole a little closer to Earth is anyone's guess, but hey, it wouldn't be as much of a movie otherwise, right?