Dark side is a bit of a misnomer, but there are two ways of looking at it. At times, some people have been of the belief that one side of the moon was always dark, hence the phrase "dark side of the moon." As NASA astrophysicist, Dr. Steve Bloom, says, "the side that we are familiar with is dark just as frequently. A better term might be "far side of the Moon."The orbit of the Moon is such that one rotation is just about as long as one revolution in its orbit around the Earth. Because of this, one side is facing us during its orbit." So, if you think of dark as meaning unknown, then yes, for centuries the far side was "dark" to us.
Of course, there is also a dark side literally. However, unlike what some may have believed in the past, it's not the same side all the time. "During a 'new moon', the moon is between the Earth and the Sun, and so the side we know is totally dark" says Dr. Bloom. "When the moon is opposite from the Sun, the side we know will be totally bright, and the far side will be dark."
However you look at it, the far side of the moon hasn't been as mysterious since the first images were sent back by the USSR's Luna 3 mission in 1959. (See First Image of the Far Side of the Moon.)
