Two words: Star Wars.
Space Opera is a sub-genre of science fiction that involves big battles, romance, advanced technologies or powers, drama, heroics, and basically everything you find in George Lucas's classic series. Although he by no means invented the category, his films are among the most iconic. One of the hallmarks of space opera is that a majority of it actually takes place in outer space, or there is a fair bit of space travel, not just a setting on another planet. So we're looking for Guardians of the Galaxy, not John Carter. Beam me up, Scotty!