YouTube copyright infringement (UMG) but song isn't released yet?

Ok so this song got leaked.. everybody had it all over the free download websites and everything.. about a month later all the links taken down and no one had the song.. I uploaded it to YouTube and Universal Music Group blocked it for copyright infringement. the song was never officially released. How do you know that it's actually copyrighted? As far as i know it was just recorded in a studio.. Is there any proof of copyright is my basic question?