Darkness can mean many things, just depends on who you may ask. Yet it does have a tendency to scare the "Sheep" (sheep is a metaphor for those who cannot guide themselves yet must be guided by another. This is most of the humans in the word). Below is my description and opinions about the "Darkness" we all experience at one time or another.
The easiest way to explain darkness comes from the Eastern schools of thought and spirituality... the TAO. The Tao or as the less enlightened westerners call it the yin-yang, represents both light and dark aspects. The Yang is the light side on the right, representing most of all "Active" energies. The Yin on the left side represents the "Receptive" energy. These are the two sides of a battery, the plus and the negative (the yang being the plus and the yin being the negative). We have through out our history assigned definitions to these positive and negative aspects. The most popular in western culture is the the light is "Good" and the dark is "Bad". Since we live in a patriarchal society, I easily see why this has come to be. You see the light represents Male and the dark represents female. Below is a comprehensive correspondence chart of which each side represents.