In reality though, our maps of the far side of the moon are as good as those on the near side.
> The hemisphere is sometimes called the "dark side of the Moon", where "dark" means "unknown" instead of "lacking sunlight"
[ed note: compare "dark ages"]
So the "unknown" (relatively anyway) side of the moon is less mapped, but not because it's unlit, but because it's dark. The lit side of the moon is well mapped, not because it's always lit - sometimes it's unlit, but it's certainly the known bit as we've been looking at it since before we were human.
Tbqh I think "dark side of the moon" is more often a genuine misconception than a poetic way of saying unknown. Fundamental misconceptions of the Moon abound, such as the mistaken belief that the Moon is opposite of the sun and rises when the Sun sets. You'd think that people would notice the Moon in the sky during the day for much of the month, but some people just don't pay attention to the sky, don't think about it much, and pick up the meme of the antisolar synchronous moon from cartoons, video games and grandfather clocks.
And sometimes people simply mispeak, perhaps due to Pink Floyd.