I would not consider darkness an element as much as I would consider it a representation of something that encompasses every element.
Darkness is, as Awake said, the absence of light. Light in most symbology represents life, the awareness and knowledge day brings, sight, and in the simplest respect the sun (which is a ball of fire). Darkness on the other hand, being the absence of light would be more in line with death, rest, recovery, intuition, the night and the moon. You can see this separation yet unity in the Underworld which is usually portrayed in the darkness sense, and the Upperworld which is usually portrayed in the bright sense. All of this is connected by a tree, web, etc. Even "white" and "black" magick demonstrates the idea of "light" and "dark". Magick has no color, so the portrayal doesn't exist in reality, but it is labeled such because it is how people view it. The truth is, one cannot exist without the other ;)
Both darkness and light can be seen within all of the elements, within all of nature, and within ourselves. They are a lot like chaos, order, spirit etc. These are not elements as much as they are forces within the universe that reside in everything. They cannot stand alone therefore they are not an element.
The Chinese elements is a whole different philosophy. Don't get me started on that. You have to know the philosophy to understand the weird ones like wood. They see it as natures/behaviors not literal like the ones most indigenous tribes and Greek philosophy supports. My advice to you is do not mix Eastern with Western philosophy. It does not mix. You can learn them both, but do not mix them.