This is a question that's been asked to greek philoshopher Democritus around 2 and a half thousand years ago. Democritus is known as the formulator of the Atomic Theory for the Universe, and the theory of 8 basic elements in Nature : It is 4 against 4, like two armies in war : Water, Soil, Ice, Darkness vs. Electricity, Air, Fire, Light. In today's world in 2013, all the 7 water, soil, ice, electricity, fire, air, light are controlled and can be manipulated easily. But Darkness!!! Because the Bible and all the other God's Holy Scriptures ban the use of Darkness, which is a matter of casting spells and doing bad things, Human Societies who believes in God don't use, don't control and don't manipulate Darkness for more than 2500 years, all because in the Holy Books, its written that it will only bring calamity and bad luck to you if you use the powers of Darkness.
So as a result: When we see the Darkness running away from Light, when switched on, it means that Humans use the powers of Light, against the stable Darkness. But think about when does actually Darkness uses its powers against Light? A normal person who believes in God, or who doesn't believe in God(but doesn't fight with God) , can't see the Darkness in active use. But a magician, a clairvoyant, a psychic or whatever, they are vulnerable to attack by Darkness.