Another possibility, albeit one not terribly popular with science-fiction writers, is that interstellar travel is just too difficult. There is no way to travel faster than the speed of light, no wormholes or “jump points”. No ceramotitanumsteel alloys or materials that will hold up to the stresses of journeys of hundreds or thousands of years. Interstellar travel is not something that complex organic beings can survive in a closed system for the amount of time it takes to cross the stars, and artificial intelligences can not handle it either.
In short, interstellar travel is an engineering problem for which there is no solution.
What implications does this have for your fictional world? Has humanity accepted this, or are they still trying? If they are still trying, how? Genetic alterations, bigger and bigger colony ships, heavily-shielded embryo and memory storage? If they have accepted it, what has the end of exploration done to human culture?
While using this explanation for the lack of aliens rules out a lot of traditional space opera and military science fiction type stories, it still leaves a lot of room for hard science fiction, and stories exploring social and political ideas. Here are a few potential conflicts or backgrounds for stories with this explanation.
- Colonies on the moon declare their independence from Earth, putting the planet under a permanent blockade to preserve the resources of the solar system for their own descendants.
- Genetically altered humans bred for and forced to crew almost certainly hopeless interstellar missions.
- A world where mass sterilization has been enforced, and only the fortunate few are allowed to procreate.
- A far, far future world where what is left of humanity ekes out an existence on a resource-drained world.
- An artificial intelligence that has hijacked Earth’s industrial capacity in futile attempts to launch copies of itself to the stars