Jus like Magic is present in all the Skylands (Skylanders are ALL magical beings, Tech ones included) I think Light and Dark might have also influenced the other Elements, a long time ago.
For example I tend to think Dark "created" or helped forming Fire, Earth, Undead and Tech since they are Elements I think revolve more around destroying, corrupting and negatively influencing the environment or the creatures.
Conversely, I associate Light to Water, Air, Life and Magic because I think about them as the "positive" Elements (I know a tidal wave or a hurricane can be as devastating as a volcano but as they are in nature water and air aren't dangerous like fire can be) the ones that are strictly connected to all the living beings.
Long story short: I think Dark exists within all the Undead, Fire, Earth and Dark Skylanders, just like Light is embedded within the remaining four.
In my opinion they are some sort of pure or primordial Elements from which all the other 8 originated.
After all, the Elder Elementals which I assume to be quite ancient "possess the powers of Light and Dark within their flesh".

What do you think?
