It seems a vast majority of humans are afraid of the dark, or have been afraid as kids. It’s so common, that there’s even a wikipedia page on this topic. There are also several theories trying to explain why children (and adults to some extent) have this fear. From the wiki page, it appears that Sigmund Freud and some researchers consider this fear a manifestation of separation anxiety disorder (SAD). Of course, this would lead to the question of why some or most people would be axious about separation. I won’t go into that question at this point, because I believe the roots of this fear of darkness lie elsewhere.

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This hypothesis is based on my observation of my daughter, who was not afraid of the dark when she was  younger, and when she could not move around on her own. When she learnt to walk, it gave her greater freedom to go where she wishes to be and do what she chose to. It is this new found freedom that is threatened by darkness. And it is this fear of being constrained, of being stripped of our choices and liberty, of losing the power to take control of our life that makes children fear darkness. As they grow up, just as when civilization progressed, they find ways to overcome this handicap (use a torch, for example) and so the fear grows weak.

So, in my opinion, the fear of dark is more related to claustrophobia than to SAD. These days people are so dependent on internet and cell phones, and these have so much come to control their ability to express and influence, one might say that the way we feel about a network blackout is more or less similar to what a toddler feels about being left in pitch dark.

Without even considering the psychological roots of this fear, there is a good reason why it has been strengthened through the process of natural selection as our species evolved. At a time when humans were still hunter-gatherers and used to share their habitat with dangerous wild animals that have a better vision and thus a natural advantage at night, brave men who ventured out of their camps in the dark were unlikely to have been able to pass on their genes. Clearly, it is not surprising that most children and many adults today have this irrational fear, because this could be a survival instinct that we genetically inherited from a time when there was nothing irrational about this fear.