The trick about building good fictional worlds isn't to make them realistic but to make them internally consistent.
Internal consistency means:
- The world is based on certain rules, which might or might not be the rules under which our own world operates
- The rules of the world are followed consistently throughout the narrative
- The history and current state of the world appears plausible under the implications of those rules
- The characters act in ways that make sense within the context of the world they live in
Let's take one of the most popular science fiction franchises of all times: Star Wars. Is it realistic? No. Some characters have supernatural powers that defy any scientific explanation. Space vessels travel at the speed of plot. The larger ones act like ships and the smaller ones like airplanes. Artificial gravity is everywhere. Single biome planets. Laser swords that somehow behave like solid swords and can deflect energy weapons. All of that makes no sense at all from a scientific perspective.
And yet the universe has a huge fandom. Especially among scientifically inclined geeks who should know that none of that makes sense. Why? Because the universe is internally consistent. There are rules how the universe works, and those rules are (largely) followed throughout SW canon. Any audience member who is willing to suspend their disbelieve and accept that the universe operates under those rules will have a great time experiencing stories which probably wouldn't work in our boring reality.
But when some new narrative tries to stray away from what was previously established in canon and appears to break those rules ("Midichlorians") then you get fans up in arms. Not because the new element isn't realistic, but because it breaks the previously established consistency of the universe.