Science-Fiction mainly focuses on currently-known principles of science, or some reasonable extrapolation thereof. Faster-Than-Light travel is an acceptable "gimmick" if the story requires it. Otherwise, no magic or psionics, no spiritualism, no supernatural realm, and no 'god' or 'devil', except where Clarke's Third Law applies:
"Any sufficiently advanced technology is indistinguishable from magic."
Ghosts? No ... holograms.
Zombies? No ... robots.
Monsters? No ... mutants.
Demon Possession? No ... schizophrenia or "brain fever".
Magical Items? No ... advanced alien technology.
The fantasy genre does away with strict adherence to scientific principles, and does not even try to explain things like magic, ghosts, and so forth. Instead, fantasy writers rely on common tropes to provide the background for their stories.
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The mere fact that science may not yet adequately explain an object, event, or experience does not mean the immediate explanation should automatically default to a conspiratorial, extraterrestrial, paranormal, or supernatural cause.