Earth 88

Since time immemorial, the heirs of the Builder and Explorer clans have been traveling along the length of the great star cable, ever searching for the end of the road, that some fear may never come. Every so often, they encounter societies of creatures that are like them, rest stops of some mysterious precursor species. And every time they encounter one of these villages or nations, some choose to remain behind, while the more determined others forge on, joined by the more adventurous of those from the society they pass. On occasion, some would stop and form a town when they wanted to rest more permanently, but in these cases, there are still those who determinedly go on and on.
The Star cable itself is unusual in that it is like a long tube in space, which somehow manages to provide all that life and technological advancement requires throughout the entirety of its length, despite being no larger than 30 miles in diameter. 
When standing on the exterior surface, one can see the vast, innumerable stretches of stars in all directions. Those of the explorer mindset often do so to remind oneself of one's calling to explore the vast unknown that exists further and further on in the star cable. On the inside of the hollow cable is an environment that manages to have a proliferation of fruit-providing plant life, many wild animals, rivers and streams of mostly drinkable water, consistently breathable air, and gravity that points to the circular interior walls. 
The gravity seems to come from the spinning of the great cable. Those dwelling in the cable also experience day light cycles. It is theorized that the light is emitted somewhere at the end or in the middle of the cable and is turned off and on every 12 hours or so. The idea is that the light must have been emitted years ago before it ever reaches any given part of the cable. 
One can also survive walking on the exterior of the cable, but it gives many dizzyness and nausea, not to mention air being somewhat thinner there (like walking on a tall mountain on earth). Thinkers have often pondered how such a thing can exist, as it is too weirdly suited for humans to occur naturally. Some theorize that some technologically advanced precursor had made it, but who would do so and why remains a mystery.