If fire is light, then I like the idea of darkness being water, as 1, they are oppisits, 2. Water seems more like darkness to me then earth. Although, I agree with cucooruler, it would be simplier to just put them as their own thing. If fire is plasma, air is gas, earth is solid, water is liquid (although technecaly, air and water are both fluids), light can be spirit, and dark is 'dark matter' if we are going by forms of matter.