Unconscious imagination of sorts... A product of infinity. It does not exist as a tangible thing because you could never have a tangible object without it being granted some type of form by a brain's perception.
So it is an existent nothing that can only be described with math... Very strange, but true... Play around with the semantics if you want.
If you want to know what an existent nothing is like, use your eyes to look at the UV rays currently beaming onto your skin. They're there as you know. That is the essential nature of matter.
