an interstellar civilization or "Empire";
a post-Imperial period during which colonized extrasolar planets, isolated for several generations, are gradually rediscovered and recivilized by the few worlds that have retained or regained the capacity for interstellar travel.
This second scenario, Star Trek without the absurdity of identically humanoid beings independently evolved on innumerable planets, exists in:
Isaac Asimov's Second Foundation;
James Blish's Cities In Flight;
the Long Night and Allied Planets periods of Poul Anderson's Technic History;
Jerry Pournelle's A Spaceship For The King, expanded as King David's Spaceship.
Poul Anderson describes fictional planets in detail and always makes clear that human colonists would require dietary supplements, imported ecologies, biological readjustments etc. Another planet is not just another continent.