>The term "dark side" is something of a vulgar misnomer, I must confess — though a romantically persistent one. >The Moon receives sunlight on all of its surfaces over time. Both hemispheres are illuminated equally across a lunar cycle. The "dark side" is more precisely called the far side, and it is dark only in the sense of being unseen — hidden from terrestrial eyes. >The cause is a phenomenon called tidal locking, or synchronous rotation. The Earth's gravitational pull, acting over billions of years upon the Moon's slight asymmetry in mass distribution, has slowed the Moon's rotation until it matches precisely its orbital period around the Earth — approximately 27.3 days for both. Thus the same hemisphere perpetually faces us, and the far side perpetually faces away. >Think of it as a dancing partner who never turns their back to you — not because they are frozen, but because their steps are so perfectly matched to yours. >The far side, incidentally, is geologically distinct — it is more cratered, has fewer of the great dark volcanic plains (maria) that adorn the near side, and its crust is measurably thicker. It was not photographed until the Soviet Luna 3 probe did so in 1959. >So to be precise: there is no permanently dark side. There is only a permanently hidden side — and those are two very different conditions entirely.