In common parlance, the Moon has “a dark side.” This doesn’t mean that the backside of the Moon harbors a secret base for Darth Vader where he and his minions practice using “the Force” for evil. Rather, it refers to the fact that one half of the Moon is visible to us on Earth and the other hemisphere is never visible. Saying that this unseen side of the Moon is a “dark side” is not correct — it actually is no darker than the side that we see — sunlight falls equally on all sides of the Moon. It would be more accurate to say that we don’t see the “far side” of the Moon.
The reason we only see one side of the Moon is fascinating. It’s not because the Moon doesn’t rotate — it does — it just rotates at a speed that is the same as the amount of time that it takes to orbit Earth which is 27 days, 7 hours, 43 minutes, and 40 seconds. This is called synchronous rotation Here’s an animation from NASA showing this phenomenon:
If the Moon didn’t rotate on its axis we’d see the entire Moon rotate past us over about a 30 day period.
It would seem to be a coincidence of cosmic proportions that the Moon rotates on its axis in the exact amount of time that it takes to orbit the Earth. But it’s not a coincidence — all the major moons in our solar system only show one side to their planets. It happens due to something called “Tidal Locking” which occurs due to the gravitational interaction between the Earth and Moon. Tidal locking happens because the force of gravity between a planet and its moon(s) is so great that it causes the moon to bulge as it rotates past. This bulging is sort of like creating handles that slows moon rotation until it tidally locks into a rotation equal to the time it takes to orbit. If you are interested you can read more about the phenomenon here: Tidal Locking.
Here’s a related IFOD on how the Moon probably formed (how it happened is pretty interesting): How did the Moon form? Here’s an artist’s representation of its formation:
Also, it is good to know that Pink Floyd’s 1973 album “Dark Side of the Moon” has sold 15 million copies making it the 26th best-selling album of all time.