What are the causes of dilation of pupils?

A few things: I assume you mean dilated and not responding to light. This can be caused by a number of drugs including scopolamine, atropine, most hallucinogens and many others. These should cause the pupils to be dilated equally (unless directly applied to one eye). If your pupils are not equal, then we start to get worried about problems like a head injury and you should seek immediate medical attention.

Darkness: Lack of light is the stimuli for pupils to dilate so more light enters the eyes. Other causes include drugs, poisons, neurology diseases. If both pupils are same size and react to light equally then i would not be concerned.