There's a lot of speculation going about this "sixth element" thing. Here's my take:
There is mentions:
"The five elements are, in ascending order of power, 地 (tsuchi, do or chi) Earth, 水 (sui or mizu) Water, 火 (ho, ka or hi) Fire, 風 (fū or kaze) Wind, and 空 (kū). This last is usually translated as "void" when referring to the elements, but refers to "sky" in most other contexts, and is therefore sometimes translated as "Heaven". Sometimes a sixth element is added, representing Consciousness, or 識 (shiki)" - taken directly from wikipedia
this implies that while there are 5 main elements, there are sometimes a SIXTH added element, which would go kind of well with the whole Japanese mythology thing. Note that Void is the fifth element here.
On the other hand, the "void" element in the five mentioned above is replaced by lightning in the manga. Possibly, this is because there would be no clear definition of elemental weakness or strength if void was in between. So, it would be possible that it was pushed to the sixth element, because does not have any clear form or definition. We all know what fire, water, wind, earth, and even lightning is, but void seems somewhat more mysterious, and undefined.
Then again there is the consciousness thing above, which probably means an elevated state of mind (also refer to "Void" below) which fits with Pein's supposed god-like understanding and growth (of the mind).
Kū, most often translated as "Void", but also meaning "sky" or "Heaven", represents those things beyond our everyday experience, particularly those things composed of pure energy. Atoms, their component particles, and atomic forces fall under this category, as do people in a higher state of consciousness. Bodily, kū represents spirit, thought, and creative energy. It represents our ability to think and to communicate, as well as our creativity. It can also be associated with power, creativity, spontaneity, and inventiveness.
Kū is of particular importance as the highest of the elements. In martial arts, particularly in fictional tales where the fighting discipline is blended with magic or the occult, one often invokes the power of the Void to connect to the quintessential creative energy of the world. A warrior properly attuned to the Void can sense his surrounding and act without thinking, and without using his physical senses." - also from wikipedia
"people in a higher state of consciousness" sounds a lot like Pein, doesn't it?
I don't think light or darkness is the sixth because they have to co-exist, and cannot stand alone. It's also somewhat represented by Yin and Yang chakra.
actually, in the western concepts of elements, "void" is considered the fifth, as there are only 4 main elements, but then again, this revolves more around Japanese ones, so I'm not going there.
Also, there are some hints as to the number 6 here. Take for example:
The founder of the ninja world was known as the "Sage of the Six Paths". There's a possibility that these six paths refer to the six elements. Also, Pein has six different bodies that he can switch between.
Besides that, notice that Pein has six concentric circles around his eyes, three in each eye. This is, of course, his Rinnegan, so the circles probably have more to them such as the elements. Not to mention that there are 3 tomoe in each Sharingan, adding up to six total.
edit: my bad, he has 4 circles around his eye. It's a bit hard to catch because you usually only see 3. Sorry.
There is also the concept of the "six realms" in Buddhism. Rinnegan translates literally into "Samsara eye", which is in connection to the six realms
the six realms are:
- the Deva realm
- the Asura realm
- the Human realm
- the Animal realm
- the Hungry Ghost realm
- the Naraka realm (hell)
there's the possibility that his eyes represent a connection to this. these could be the six paths. note that all are associated with reincarnation, so that plays well into his body-switching concept as well.
Anyway, it looks like I'm going off-topic. These can all point to the number six, so it makes sense to have the sixth element as well. It's said that all ninjutsu were derived from Rikudou Sennin's abilities. It's likely they meant the elements. Kakashi does state that the elements are the foundation of all jutsu, when he teaches Naruto nature manipulation.
I've come up with a way to represent Void in accordance with the elemental weaknesses and strengths. First, let me refresh your memories:
Fire beats wind,
Wind beats lightning,
Lightning beats earth,
Earth beats water,
Water beats fire.
So to me Void is like the colour white; broken up, white splits into the seven colours of the rainbow, right? It's somewhat the universal colour, and fits perfectly in general terms. The colours as opposed to elements might be represented in the colour wheel, or rainbow, and white, though not seen in either, plays a role as a combination of all of them (at least the seven)
So void is rather the same. It is the white of chakra (I don't mean literally white, but you get the picture),and all the other natures are "combined" (in a general sort of way, not necessarily chakra combination) to form Void.
Think of it this way: if each element eliminates the weaker one, then essentially they cancel each other out: fire beats wind, wind beats lightning...until water beats fire at the beginning. And what do you get? Nothing. "Void", essentially.
So that's it for now. What do you think?