Inner Space vs Outer Space
Much of Science Fiction takes place in Outer Space. Boundless adventure! Endless opportunity! Formidable challenges! There are so many opportunities for exploration, for intriguing storylines, for villainy and heroism. Space is simply the best.
Many Science Fiction stories revolve around the idea that at some point in the distant future the human race will have conquered the entire galaxy. Our descendants will then inhabit millions-- or billions-- of planets. In some versions of this vision the inhabited planets will constitute a massive galactic Empire ruled by some central administration under the leadership of a single galactic ruler.
Most science fiction stories depend on the concept of faster-than-light travel. The reason is simple-- the galaxy is immense. The central plane of our galaxy is about 50,000 light years in radius, and it has a halo of dark matter and globular clusters extends another 100,000 light years beyond that. The Earth orbits the galactic center at about 27,000 light years; the circumference of the Earth's orbit is therefore roughly 170,000 light years. The Earth traverses this immense distance once every 225,000,000 to 250,000,000 years. Hence the Earth is rotating about the galactic center at about 0.07% the speed of light.