Darkness is associated with evil, ugliness, scary monsters, and super creeps. This is the reason for the naming of The Dark Side and why Evil Counterpart characters and certain Underground Monkeys often have 'dark' in front of their names. The logic behind the trope is as follows: most humans fear the dark, at least to some degree; our sight is the sense we depend on the most, and we cannot see well in darkness, therefore a lack of light makes us feel very vulnerable to danger.