The same side of the Moon always faces the Earth. The "dark side" is not actually dark — it gets cycles of day and night just like most places on Earth — the "far side" is a more correct term. The reason that one side is never visible from the Earth is because it spins once on its axis in precisely the same amount of time it takes to revolve around the Earth. If its rate of rotation were slightly different than its rate of revolution, those on Earth would eventually be exposed to the entire surface of the Moon. These two intervals have been equal for all of recorded history, however, and probably have been for millions of years or longer.
This otherwise bizarre phenomenon can be explained in terms of a subtle effect generated by gravitation and friction called tidal locking. Through their mutual gravitational attraction, the Earth and the Moon create tidal bulges on each other, with one bulge facing in the direction of the other body, and one facing away. These bulges generate heat through the friction of rock rubbing against itself, and also change into a greater orbital force for the Moon, which means that it is continually getting further away from the Earth. Over time, they siphon energy away from the rotational momentum of both bodies, producing a braking effect.
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Because the Earth's mass dominates the Earth-Moon system, the Moon experiences the greater braking effect. Over time, its rotation has progressively slowed until the rate of rotation matched the rate at which the tidal bulge moves around the body. Today, lunar tidal bulges are located at a constant position with respect to the rotation of the Moon, meaning a sort of equilibrium has been reached.
The Earth's rate of rotation also slows over time due to the tidal forces, but the braking effect is much smaller — in order to be synchronized perfectly with the Moon, the Earth would need to rotate only once per lunar cycle, or about every 29.5 days. Then the Moon would always be in the same place in the sky, and visible from only one side of the Earth, but this is not the case. In certain planetary systems, like that of dwarf planet Pluto and its satellite Charon, both bodies are tidally locked to one another.