nothing
In physics they can't even see the atom. Only the effects of one.
When an item or even space is broken down into it's material molecules or knowns, the weight of the parts do not equal the weight of the whole.
There is something else there in the whole which is nothing.
Even if you were to be broken down into your little chemicals and what nots, your bits wouldn't equal you as whole. So as a whole, you are made of somethings and nothings.
LOL...it's heady physics. Not to mention very Tao.
Both nothing and something have to exist for everything to be here.
I highly recommend that book.