“It’s important to remember that the term “space opera” was first devised as an insult.
“This term, dropped into the lexicon by fan writer Wilson Tucker, initially appeared in the fanzine Le Zombie in 1941. It was meant to invoke the recently coined term “soap opera” (which then applied to radio dramas), a derogatory way of referring to a bombastic adventure tale with spaceships and ray guns. Since then, the definition of space opera has been renewed and expanded, gone through eras of disdain and revival, and the umbrella term covers a large portion of the science fiction available to the public. It’s critical opposite is usually cited as “hard science fiction,” denoting a story in which science and mathematics are carefully considered in the creation of the premise, leading to a tale that might contain more plausible elements.
“This had led some critics to posit that space opera is simply “fantasy in space.” But it isn’t (is it?), and attempting to make the distinction is a pretty fascinating exercise when all is said and done.”
— Emily Asher-Perrin, “Is Space Opera Merely Fantasy Set in Space?” in Tor.com
(worth a look at the article for the backcover of the first issue of Galaxy)