Why does dark chocolate turn white after being in cold for some time?
Its called chocolate blooming.

There are two types:


Fat blooming - cause is not known for certain, but probably the type VI chocolate crystals are more thermodynamically favored, so spontaneous conversion (and exit from the surface of the solid solution constituting the chocolate body) may be possible.
Sugar blooming - the sugar in the chocolate is dissolved by humidity, and the precipitates on the surface.


Fortunately, there are no safety problems with bloomed chocolate, and it can be remelted with no inherent loss of quality.   So bloomed chocolate is perfectly useful in recipes where it will be melted; or it can be melted and re-tempered if desired.