Why can I see the dark side of the moon? Last weekend (29/30 April) the sky in my area (Central Europe) was pretty clear and the Moon was nicely visible. Funnily, one could not only see the sunlit area of the Moon, but also the dark side* could be easily recognised throughout the whole evening from blue hour until dark night. I attached two photos of the Moon that should illustrate the effect. The pictures (sorry for the poor quality) needed to get a little overexposed to capture the illuminated dark side, but with plain eye the effect could be well seen.
We were speculating about the reasons why the non-sunlit area of the Moon could be seen.


diffraction of sunlight through the atmosphere can be neglected as the Moon doesn't have an atmosphere. 
reflection of light that in turn was reflected from earth (Sun -> Earth -> Moon -> observer) sounded a little unlikely to me
a psychological effect that kind of mentally completes the shape of the Moon, because we know it should be circular, sounded ok to me. But could be debunked as the dark side stays visible if one covers the bright side on the photo? 


So I guess the  sunlight has to reach the dark side of the Moon somehow differently.

What could be provide enough light to make the dark side visible?

* not the far side of the Moon, but the non-sunlit area of the Moon
You can generally see the unlit side of the moon when a considerable amount of sunlight is reflected off the earth. This reflected sunlight illuminates the unlit side of the moon. This is referred to as earthshine, and a decent explanation can be found at timeanddate.com. I seem to recall reading somewhere (but now can't find a reference) that it is more prominent when it is very cloudy on earth near where the moon is overhead.