Why is there a "dark side of the moon  

One side which block the sunlight is the bright side of the moon. obviously the other side has to be black.
This is a nice discussion. Keep going, byee. Wish u r payout soon.

this is because The Moon rotates exactly once per revolution of the Earth which is the reason why it shows only one face at the Earth. If you face a pole and move around the pole showing your face at the pole all the time, you will notice that you have rotated once per revolution around the pole.

Why has this happened to the Moon? It is due to tidal forces of the Earth. You know that the Moon's tidal forces causes the high and low tides on the Earth. In the same way, the Earth exerts tidal forces on the Moon which are more powerful as the Earth is more massive than the Moon. It turns out that these forces exert torques on the rotating system and tends to slow its rotation till it finally shows the same face towards the other body. Hence, it is the effect of tidal forces of the Earth on the Moon that have caused the Moon to show only one face to the Earth.

Similarly, the tidal forces of the Sun on Mercury have slowed down its rotational period to once in 59 days. The Moon's tidal forces will have the same effect on Earth, so that some day in future (billions of years hence), the Earth may show the same face to the Moon.

You can also view video explanation from NASA site > http://www.nasa.gov/audie...

actually, sachin075in has it partially correct. the scientific fact is that the earth's center of magnetic polarization is such that it aligns with the iron core of the moon that happens to be slightly off center, which keeps it from spinning on an axis. it's drawn to our magnetic balance. yes, it only rotates once ever 27.78 days (at least i think that's the time frame), but were it to rotate any faster, the gravitational pull would have devastating effects on the continental shelves, tectonic plates, etc. in short, we could be come neighbors with africa and japan in a matter of a few millenia rather than a few million years. it's scary stuff, that moon thing.

because if there wasnt there would have been no pink floyd album