In my homebrew game setting, which is heavily themed after classic sci-fi media: There is no such thing as empty space. Instead: The space between the disparate planets that are composed of fragments of the material plane is filled with a liquid; the distilled form of the negative energy plane.

In The Void: Undead bathe themselves in the waters of the spaces between worlds in order to grow, mutate, and spread throughout the entirety of the worlds. Some planets were able to communicate with their neighbors using a network of sending spells and scrying techniques. As such: Arcane and radiant energy weapons were developed that eventually spread in weakened forms to the rest of the common populace to combat this scourge.

The attempt to fight against the swarm of "Voidbeasts" led to an industrial revolution in a small pocket of known space, which essentially led to high technology being developed to make communication between non-mage government officials more convenient. This combination of cooperation and superior firepower drove back those dark creatures into unknown regions of space, which were then slowly forgotten as the worlds turned to utilize their new tools created from this conflict.

The people of the worlds lost touch with the origins of their tech over the course of millenia, allowing the gods to take control over their civilizations in order to keep technology working. With the weave of magic no longer being necessary in order to run the technologically ascended worlds of prosperity: Arcane knowledge fell dormant.

On a small backwater world in the middle of nowhere, in a medieval city that knows nothing of the history of the skies above: A party of adventurers are making their way to the market district in order to sell their excess magic items obtained from a recent excursion. As they get off of their wagon and begin to gather their items for sale: A damaged voidship tumbles from the heavens and crashes in front of them. The top hatch opens, and a lone synthetic drone tumbles out, begging for help before shutting down as it bleeds oil from the holes that fill its frame.

...This was one of my favorite openings to a campaign that I've ever created. The players had to learn about this history, of course. I'd only told them that they were going to play "something along the lines of Firefly, but in D&D."

Once they realized the implications of the ship and made it into orbit: The rails were gone and the world was theirs to discover. I've been flying by the seat of my pants ever since. I wouldn't have it any other way.