Science Fiction novella by Robert Heinlein (and no part of his Future History). * * * * 1/2; simply one of the best stories ever written.
Man's aspirations to reach the stars had its fruition in a generation ship, a ship large enough to provide for all the needs of its passengers, a ship in which people would pass their entire lives travelling from Earth to another star, a ship that formed their entire Universe.
The trouble was, the travelers forgot they were travelling. We enter the story long into the history of the voyage; the outer shells are uninhabited because of a long-ago revolt. Physics and metaphysics have become inseparable: Without the visual laboratory of the sky, science becomes a matter of faith. Priests interpret the ship's engineering diagrams for the faithful. The story's young protagonist challenges the priests and is exiled from society. To vindicate himself, he begins a quest for the mythical Control Room.
First appearing in Astounding Stories in 1941, "Universe" is a signpost in the history of Science Fiction: In an era when most authors had their spacemen travel the forty billion or so miles between inhabited star systems in half an hour, Robert Heinlein was coming to grips with the immense distances between the stars, the impossibility of faster-than-light travel, and suggests a possible way to overcome those obstacles. Even though it didn't work.
Do not dismiss "Universe" as simply another nuts-and-bolts hard science fiction story (although it's probably one of the first ones). It is more about the way The Law of Unintended Consequences sends the best-laid plans awry, and how silly it is to make several generations of one's descendants follow a predetermined path.
Heinlein followed "Universe" with a novella named "Common Sense"; these have been collected into an omnibus volume titled Orphans of the Sky.
"Universe" has left its echoes1 throughout the future development of Science Fiction. Your quest, now, is to find the story and read it.
1Well, pretty much everything that followed it, but more directly things like Ringworld, Red Dwarf, Flux, 40,000 in Gehenna, Rendezvous With Rama. More to come as I think of them.