It's a joke about perspective, not astronomy.

The line is literally true from one angle, but the whole point is that it sounds profound but is actually a contradiction. The “dark side” we talk about is the side facing away from us, yet every part of the moon gets sunlight eventually. Calling it “all dark” is just playing with words — if you’re in the shadow, it’s dark everywhere except where the sun hits. The album uses this to set up its theme: we project darkness onto things, but it’s all relative. Nothing is purely light or dark.

The quote comes from a pub door attendant Gerry O’Driscoll, not some philosopher. Pink Floyd asked random people questions and stitched the answers into the album. That makes it even better — it’s just a guy stating the obvious with a twist, and they turned it into one of rock’s most famous intros.

You’re not supposed to overthink it. Enjoy the sunlight while it lasts.