I don' think there really is such thing as shadow element. There are mainly 5 to 4 elements.
I mean shadow could possibly be placed underneath spirit element. But i don't think its real. I mean this is my opinion only. But i haven't really found any information. But those are main for elements. There really is no such thing as ice, electric or shadow elements.
There are more than 5 elements.
Elements are the makeup of matter.
Anything that is matter is part of an element. There are the main 4/5 (depending on your belief), and then there are subsidiary elements such as weather.
Light and dark are arguably elements, as neither is actually composed of matter like water and fire are.
But just as with everything else, it can depend on personal belief.
Brysing can think that only fire, earth, water, air, and spirit exist within the universe if he so wishes; and so can Lunar.
Although if you're looking at it from a less spiritualistic bias, then there are many many more than just 5.
Known states of matter: solid, liquid, gas, plasma (super hot), and enstinium condenate (extremely cold to the point where matter behaves in a different fasion).
Hydrogen, helium...I don't want to list all of the know elements.
Humans are obsessed with categorizing things. I say that you never have just one pure 'element', its always a mixture. However, shadow is merely the lack of light, so I don't think it would classify as one anyway.
Magick is spiritually based so any one looking at it simply only physical is missing half the point, this universe is made not of only matter, to determine elements you simply look at the primordial composition of the universe and its sub sections, for eg, a large part of the universe is darkness referring even into dark matter which is very advanced occult work for those who are familiar with darkmatter , light is actually also part of it, but not just acting as an opposite (since polarity does not exist on these scales)
Spirit of course undoubtedly is a major part of the universe having its counter part ether which is literally space and sound. Ether is extremely important cause without it no other element afterward could exist properly. Ether regulates the "sacred space" of existence of all things hence having a various subset of one divine energy which creates elements fire, air etc. I do hope im not being too heavy and confusing anyone but i just needed to get out to people the universe's aspect view is not limited by you, your view of it is limited only by yourself, there is much to this world that meets the eye
Yes, a large portion of the universe is dark matter, but it doesn't have to do with actual darkness. It is called dark matter because of the lack of knowledge about it. Intelectually dark. The only reason we know it exists is because there is an unexplainable gravitational pull towards the matter. We do not know anything more on the subject in science.
However, yes, like you said pureindigo, there are the spiritual, nonphysical categories that we cannot simply ignore.