The Moon does not have a dark side. It keeps the same side pointed towards the Earth. That side is either illuminated and and hence fully visible at full moon, or completely unlit and therefore invisible at new moon. There are of course all the intermediate stages in between - the phases of the Moon.
Incidentally, because the Moon "wobbles" a little on its axis it is possible to observe some 55 per cent of its surface from Earth throughout the year.
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