Despite popular belief, the term "space opera" has nothing to do with that scene in The Fifth Element when the blue lady sings. Sorry to disappoint.
Instead, space opera is a sub-genre of science fiction that first emerged in the 1930's. Although these early works utilized the advanced technological settings of classic sci-fi, they abandoned the scientist's obsession with accuracy in favor of melodrama, adventure, and grandeur. In other words, it wasn't exactly high art. In fact, space operas were seen as "pulp fiction" at the time (which doesn't have anything to do with John Travolta, and is instead an insult to the book's quality).
That's why it's not quite right to call Star Wars "science fiction." After all, the revelation in The Phantom Menace that Force abilities are caused by microscopic organisms caused a huge uproar in the Star Wars fandom, because it invalidated the mystical bent of the early films. (You wouldn't hear someone complaining about too much scientific detail in a Star Trek movie, would you?)
That's because The Empire Strikes Back is a classic example of fantasy. Sure, it features advanced technology, but what we're dealing with are mythical archetypes, not super-complex scientific truths. There are even swords, for Windu's sake!
This makes a lot more sense when you realize that Lucas was heavily influenced by Joseph Campbell's famous studies of mythology, leading him to recreate ancient, mythic structures in Star Wars.
You know who else was obsessed with recreating myths? A dude by the name of J.R.R. Tolkien. Although it uses the tropes and thematic elements of science fiction, The Empire Strikes Back is as much of a fantasy tale as anything from The Lord of the Rings trilogy.