Science fiction is difficult to explain. As Robert A. Heinlein, a science fiction author said, “A handy short definition of almost all science fiction might read: realistic speculation about possible future events.” This means that it is about realistic events that could happen in the future. In other words, it is something that COULD have happened (in the past) or can happen (in the future).
Even though this isn’t a book, here is a significant example:
STAR WARS… is a great example of science fiction. If you can recall, it says, “A long time ago…” This shows that Star Wars took place in the past. We aren’t sure if it is possible, or if it has happened, but for all we know, it could have happened, “… in a galaxy far, far away.”