a know it all
A know-it-all is someone who thinks they know everything, even when they don’t. They argue about stuff they know nothing about and act like they’re the only smart person in the room.
'I know everything about space! I watched a five-minute video about it!'
'I know more about the Civil War than your history teacher!'
'I’ve studied philosophy for one day, and now I’m telling you all of them are wrong.'