We are here to fight ignorance after all. The dark side is the side that is non-illuminated, and the far side is the side that we never see. These sometimes coincide.
Of course the far side is metaphorically ‘dark’, as in ‘hidden’, but we needed to establish what the OP was talking about. From the responses in posts #12, 13 and 14 it seems the OP is interested in all sides of this question, not just the tidal locking.
In the interest of fighting ignorance, can you provide any evidence of your assertion that the phrase “dark side of the moon” is not exactly equivalent to “far side of the moon”?
Simple: The English word “dark” does not mean “far”. The word “dark” means “not light”, and the portion of the Moon that’s not light does sometimes face the Earth.
But “dark” does mean “unknown” (sometimes), and the far side was the unknown side (until the Russian probe took pictures of that side). I haven’t find any headlines from 1959 on the Internet (yet), but I suspect some of them said “dark side.”
This has been answered (tidal locking).
Interestingly, given enough time, there will come a day when the earth is tidally locked to the moon so that only one side will face the moon. Essentially the moon will seem to hover in one spot over the earth.
That will take maybe 50 billion years (long past the death of the sun) but if the earth/moon are not destroyed when the sun expands at the end of its life it may happen.
What does that have to do with the way English works? A phrase often contains words with specialized meanings. The far side of the moon was named its dark, i.e. unknown, side because nobody could know what it looked like until a camera could be sent there. Exactly as dark matter and dark energy use dark in the same sense of unknown.
Is there any body of usage that contradicts or modifies this? Can you point to usage that involves the dark side as the unlit portion of the seeable side?
I don’t believe that any such body of usage exists outside of this thread. Note that body of usage is not a few people getting it wrong or making unsubstantiated claims like eburacum45. I checked Google Books for usage in the 20th century before the Pink Floyd album and found only a couple of hits that refer to the dark side as unlit, and those are more references to the area rather than a common phrase.
By contrast, I. S. Shklovskiĭ and Carl Sagan’s classic Intelligent Life in the Universe (1965), which I hope you have because you would enjoy it, makes specific note that “there is an unfortunate tendency to call the far side of the Moon the ‘dark’ side of the Moon…”
That’s how idiomatic phrases work in English. Idioms do not have to make either semantic or grammatical sense. They simply get weight by repetition. As do mistakes, unfortunately, which is why we should be fighting them.
No, I’m with Exapno on this.
One can certainly argue that this idiomatic usage “dark side” is misleading, and that the term “far side” is preferable.
But even if we accept that “dark side” is a poor term for the side permanently facing away from the earth, that does not imply that the term should mean (or has ever meant in common usage) the side transiently facing away from the sun. Either accept the idiomatic usage, or choose to abandon the term altogether. But it is almost willfully confusing to decree that it means something else.
Whether on the Earth or on the Moon, if the sun is not in the sky we could say we are on the night side, otherwise the day side.
If it’s an idiom, then it doesn’t make sense, by definition. But who says it’s an idiom? If I refer to the left side of my body, then I mean the side of my body that’s on the left. If I refer to the top side of a blanket, I mean the side of the blanket that’s on the top. And if I refer to the dark side of the Moon, I mean the side of the Moon that’s dark. That’s not how idiomatic speech works, but it is how ordinary, non-idiomatic speech works, which is relevant because I’m not using an idiom.
I’m pretty sure Roger Waters has seen the Dark Side of the moon.
I’m not sure about the fuss over this.
Perhaps colloquialism is better.
Usage of the phrase goes back to the 1750’s so this is not new. It is fine to point out that there is no such thing as the “dark side of the moon” but I think most people understand immediately that the phrase is meant to mean the far side of the moon (as seen from earth).
I’m not sure what you mean by “doesn’t make sense”. An idiom is a phrase whose precise meaning cannot be deduced from the individual words. But that does not imply that that the meaning contradicts the usual meaning of the individual words. It has been pointed out that the idiomatic phrase is consistent with some meanings of the work “dark”, if not the most common one.
Well, whether it’s an idiom is an empirical question. But if it is, then it’s just confusing to use it in a non-idiomatic way and expect to communicate effectively. If two canines are in conflict in your yard, and neither one belongs to you, it might not be particularly helpful to call the police and say “I don’t have a dog in this fight.”
“There is no dark side of the moon really. As a matter of fact, it’s all dark.”
Trouble is, the term 'dark side of the Moon refers to both the far side and the night side. This can lead to confusion in the minds of people who are unacquainted with the sky who might think that the ‘dark side of the Moon’ is actually dark.
If you mean the ‘idionatic’ dark side, then it is no more dark than Darkest Africa, which is pretty bright most of the time (except at night).
(Apologies if I’ve put the cat among the pigeons here somewhat, but this thread reminded me of a queston in Stephen Fry’s QI which addressed exactly this ambiguity. Most people found it difficult to differentiate between the ‘far side’, ‘night side’, ‘dark side’ and the side illuminated by the Earth.
I did not know that, interesting.
Have to get myself a telescope. 
Brilliant! 
If you use an idiom (or something that your audience thinks is an idiom) non-idiomatically, you’ll cause some confusion. If you use a non-idiom (or something that your audience thinks is not an idiom) idiomatically, you’ll cause a lot more confusion. A lot of people hear statements like “the dark side of the Moon never faces the Earth”, and conclude that only the side which faces the Earth is ever illuminated, which causes all sorts of confusion.
Should we talk about the Far Ages?