Look at this picture of a crescent Moon, taken around dawn. The bright crescent is all we can see of the half of the Moon that is facing the Sun, given the angle that we are viewing from. You'd think that the side facing the blackness of space would be dark. But you can see it because sunlight reflects from the Earth back up onto the Moon, lighting it dimly. It's like the full Moon shining on Earth - that was bright enough for us to walk in the hills last year. Now I am aware that the phrase Dark Side of the Moon doesn't mean the side out of the light - that's another story - but it sounded nice in the title!
