So when you first set your butt in the chair and decide to write Science Fiction ™, this is one of the first questions you have to ask yourself. Are we alone?
In Fantasy, in almost all flavors, the answer is “duh, no.” because you have elves, and dragons, and unicorns, and lycanthropes, and vampires everywhere, even in Urban (modern) Fantasy. In horror, the bad guy is almost always something other than human.
But in Science Fiction, when you get out into space, you have to ask: Is there anyone out there to talk to? If there are no other intelligent species to encounter, why not? Are we unique? Or have the others come and gone and left no trace? (Jack McDevitt does an awesome job, by the way, exploring this topic with his Hutch novels.)
Or perhaps they have not made it up the technology ladder (yet) and are still at some “primitive” level. Thus, the Prime Directive.
But as humans work their way into space, you as the author have to decide what they find out there. If there are other people in space, why have we not encountered them yet? Perhaps they have their own Prime Directive that keeps them from letting us know until we’re ready? Thus, the Fermi Paradox.
A theory I share with a buddy of mine got incorporated into the universe of The Collective, containing The Shipwrecked Mermaid and Imposters (so far). They are out there, but consider mankind to be dangerous homicidal lunatics best kept isolated until their either destroy themselves or grow up, if the latter is possible. We are watched. Aliens walk (and swim) among us, but they keep a very low profile and keep everything as secret as possible. Both of the Rick Pine stories that I have written so far explore what happens to that theory when something goes terribly wrong.
At the other end of the spectrum, every day scientists find new theories of how strangely unique our homeworld planet it, compared to other systems we can study, which lends itself to the theory that organized life capable of technology is likely to be fairly rare. Recently, scientists noticed how many really big planets were extremely close to their parent star. Say, Jupiter at Mercury’s orbit. No smaller planets would survive a system like that, unless they ended up as moons of a brown dwarf like Jupiter. Their current theory is that Jupiter swept in at some point and smashed up all the (smaller) big worlds in close, leaving behind just a little rocky mass from which to form four small planets and the asteroid belt.
Similarly, we have a moon that is huge in comparison to the Earth (Charon and Pluto come to mind for a comparison). Etc. Etc. Etc.
So maybe we’re alone in space. Or there are only a few other places where the conditions will be ripe for a technological civilization to occur. And of those, only a few will manage to make it safely off of one planet before disease, bad luck, or atomic weaponry end them as a civilization and possibly as a species.
This is the basis for the Alexandria Station universe. (Science Officer, Mind Field, Librarian, Greater Than The Gods Intended, Auberon, Queen Of The Pirates, etc.) There is nobody else out there.
Eleven thousand years of exploring and humans have found no trace of others. Which is not so say nobody has ever come along. David Drake is an absolute master of throwing in something obviously older than mankind, tucked away in some place where his characters find it, but can’t really explore it, and then never quite remember to come back for it later. (I love finding those Easter eggs in his books.)
We’re only a little over 100,000 years old as a distinct species. On a rock 4,500,000,000 years old in a backwater corner of the galaxy. Maybe we just missed out on someone, only a million years ago, before they came and went. Life has been around on this planet for a very long time, what would it have been like then, if someone else had evolved into the apex species other than humanity?
Perhaps those aliens left us a message out there, as McDevitt likes to explore. Or a building. Or, possibly, the great evil weapon/tool/technology thingee that eventually destroyed them. Think of Fred Saberhagen’s Berserkers, for one example.
That’s what I love about Science Fiction. I can think big thoughts, and then wrap whole story worlds around them. I have Alexandria Station. I have The Collective. I have at least two other universes I want to play in, with their own sets of rules and aliens. (I just need copious amounts of spare time with which to write. Day jobs suck, bubba.)
Because nobody knows. I can’t be wrong. I just have to have an entertaining story in an interesting place. And I get to play. Because only I get to know the truth.