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.

Related Questions

What is causing my right eye pupil dilation? I have been suffering from grand mal seizures, with severe headaches, right side only temple swelling, right eye pupil dilation, and also a sore right beside the afflicted eye.

You . You should immediately make an appointment with an ophthalmologist and a neurologist. You don't say how long this has been going on, or whether your vision is affected, but there could be very serious problems. If this just started and you cannot get an appointment immediately, go to the emergency room. Read more...
Also, . Also, other causes of abnormally large pupils can be accidental use of dilating agents (scopolamine patch, dilating drops, etc) or adies tonic pupil, or even trauma. These are rare causes. Read more...
See a physician. Seizures, headache, and a dilated pupil can be something very serious such as a brain aneurysm or tumor. Medical attention should be sought right away. Cat scans or MRI scans can help to make the diagnosis. Read more...
Need to get images. A focally enlarged pupil can be a sign of a posterior communicating artery aneurysm, and needs to be addressed asap. Hopefully this is a benign adie's pupil, but do not assume all is well, get reassured immediately. Read more...