It's a lyric from a song called Eclipse by Pink Floyd it's spoken by Gerry O'Driscoll and the full line is "There is no dark side of the moon, really. As a matter of fact it's all dark" Gerry O'Driscoll was the doorman at Abbey road studios. The next line is "the only thing that makes it look light is the sun" Apparently the lyrics are about walking towards the light instead of the darkness.Of course there is. When the main on is between us and the sun in an eclipse, the other side of it will be in full sun, so the dark side is the bit we see.