You should have asked yourself first; what is in the middle of what?
The Universe is a vast space as we know it, scientists believe that the universe started with nothing, then came a single tiny thing that evloved into different things we now called planets, stars etc., this idea is being tested at CERN with their LHC (Large Hadron Collider). But I believe this is not the answer we seek, the question that Phantasm asked was, "what is in the middle between the beginning and the end where both have nothingness?", in my own thought, my answer is there was also nothing in the middle because nothing existed in the beginning and nothing will exist in the end. And if the question is about the universe and all of its components, the middle of the beginning and the end is the present itself.