Raft - Xeelee Sequence by Stephen Baxter

The descendants of a crew of spaceship explorers, who wandered into a universe with a force of gravity one billion times stronger than today's, are still struggling for survival, The few thousand humans survive in a nebula of relatively breathable air, existing in divided communities. The society is highly stratified, with the elite living on the "Raft" (the remains of the starship that contains almost all the high technology), workers/miners living on various "Belt" worlds (where they mine burned-out star kernels), and the "Boneys", a nomadic band of "unmentionables" who live on worlds created out of corpses.

It is not directly detailed how humans came to the universe, but hints within the story indicate that the Raft ship came through a rift in our universe into this alternate reality.