Moon is the natural satellite of the earth which orbits around the earth in about 29 days.
We see only one side of moon this is because of gravitational tides. Earth and moon exert tidal forces on one another. Earth and moon are very close to each other so the earth tidal effects had used the rotational energy of moon in the beginning causing the rotation of moon to get slower and reaches to the lowest energy where moon spinning exactly once in its orbit and this facing only one side towards earth. In simple words we can say that the earthâ€™s gravity has affected the moonâ€™s rotation till it was gravitationally locked in synchronous revolution pattern. Millions of years ago moonâ€™s rotation was faster than the present times.
Moon takes29.5 days for one rotation around its axis and complete revolution around the earth because of this we are able to see only one face of the moon from every part of the earth. This can be easily understood by a simple experiment. Take a ball and mark its half with a pencil. Now assume your hand to be earth and ball as moon. Facing the same side of Ball towards hand form a circle around the hand. You will notice that it has completed its circle around earth as well as rotation around its own axis and in the whole procedure only one side faced the hand.
Same happens in the case of moon and earth. So we can say that we are able to see only one face of moon and not the other, but the other side is not dark because it also receives sunlight during rotation around earth. If the moon doesn’t rotate at all then we will be able to see the entire surface of moon at different times.
We see phases of moon because sometimes we are able to see the part of moon which is receiving sunlight and call it as full moon but sometimes only a small part of moon is lit by sun out of the whole side facing towards the earth. These are known as half moons or gibbous. But it does not matter that whether the moon is new moon or gibbous or half moon only one side of the moon is always seen from the earth.