It's called a "declarative question", and it can be used in a few different ways. The most common is when repeating someone's statement back to them: "The sky

It's called a "declarative question", and it can be used in a few different ways. The most common is when repeating someone's statement back to them: "The sky is blue." "The sky is blue?" This requests confirmation, which can imply either "did I hear you correctly?" or "are you sure?". It can also be used to request confirmation for a statement you're not repeating, like in these examples: https://www.reddit.com/r/grammar/comments/16ogm8/question_marks_in_sentences_that_arent_questions/c7y51qf/ https://www.reddit.com/r/grammar/comments/16ogm8/question_ma... But that tends to come off as presumptuous in most cases. The use here is more informal. It can't be requesting confirmation, since it's not addressed to a single person, so it's just indicating general skepticism or uncertainty. In that case it's more of a statement than a question; you wouldn't answer it yes or no.