You dig a big enough hole that water can stay in, it becomes a pond. And once it's been a pond for long enough, fish somehow appear. Even though it's not connected to any other water - hell, consider all the lakes that aren't connected to any other water. How the fuck are there fish? Where do they come from?
One day, something in space is going to look at humans the same way. You go to a new galaxy and there's freaking humans in there, like they just spontaneously manifest on random planets that have the ability to sustain them. All you need is an atmosphere with enough oxygen, some form of water, and that's pretty much all you need before they seem to just pop out of the ground and start terraforming it.
The mystery of the lake fish has been solved, by the way. It's waterfowl. Much like the birds that eat seeds and spread them around, waterfowl consume roe from the water, and while the number of fertilised fish eggs that pass through the digestive system of a duck or wild goose alive and unharmed may be small, it's not zero. A goose will shit in the lake water, and through comes the roe. It happens just often enough that lakes and ponds become - and remain - populated with fish.
Humans don't pop up on unknown planets and and unreachable galaxies on purpose. They couldn't get in there any more than they could get out of there, they have no choice but to claim the most viable-looking planet they can reach and start terraforming it. But how did they even get there?
You see, every once in a while an unfortunate human spaceship gets swallowed by a cosmic duck
This feels like a major plot point or offhanded lore dump that would show up in a Hitchhiker's Guide to the Galaxy novel.
"While the Cosmic Watefowl theory of two armed two legged one head digital watch obsessed panspermia has been largely considered a "load of Arcturan dingo's kidneys" by most accredited members of the galactic scientific community, it was unexpectedly proven sound by the discovery of a small flock of hyperducks migrating between arms of the Milky Way galaxy. In this case, the word small is only referring to the number of individuals, as the hyperducks themselves are the size of continents or, perhaps more relatably, the size of your local city councilman. Upon the humane euthanasian and dissection of one of the hyperducks, the latter by way of thirty thousand electric hyperlasers and the former by the forward prow of a yacht belonging to a small Hydranian hunting group, small in this case referring solely to the size of the beings as they are merely two Jovian apples tall as the hunting group was several thousand ships strong, each with a contingent of millions of hobby hunters, it was found that the entrails of these gigantic spacefowl were populated by a thriving civilization in the midst of a beautiful but concurrently smelly renaissance. The Greater Lord of the Duodenum, a man by the name of Paul, was unbelievably happy to interrupt his gutly principality's mass suicides in the belief that the End Times of Southward Movement had finally arrived with a tour for the visiting hunters of the lovely stomach-acid-seaside town of Gall-Way."
