The Light type.
Well, if it exist a Dark.type, so, why not making a Light type?

You are not the first person to come up with this idea.

When Dark and Steel were introduced they helped balance the type matchups, and throwing in another type now would only cause confusion as they would have to make changes to the matchups all over again. Plus, there are the same amount of Special types as there are Physical types. Adding another type would make one have more than the other.