Space opera is usually similar to earth culture. Events that happen in space can be well related with things on earth. Protagonist, antagonist will always exist. Ideology, romance, argument, and many more happens at normal cause and result. In fact, being in space doesn't change anything besides the fact that it is science fiction, therefore, being a fantasy, anything is possible.
In space opera, though being similar to events on earth, it usually involves aliens. As the world is the stage for the opera, aliens is bound to appear. Here, there will be politics as well, alliance with another alien race, or governmental order from earth to do certain things. Government matter will definitely exist.
In space opera, the boundary between good and evil is great. It is clear that the good wants to protect everything he cares while evil is overwhelmed with ambitions and domination. Space opera, being science fiction, also involves fringe science. Unexplainable things happening with no definite reasonings. Most event occurs within a space ship, or space station as it clearly representing space travelling. It is the foundation of space opera. When you mention space travel, space ship is unavoidable subject.
Star Trek and Star Wars are great example of space opera. It also has clear representation of good and evil as I explained earlier.

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