What is Nothing?

I apologise for my naiveness, but my mind is accustomed to think of things in an enclosed space. I'm unable to grasp, how "nothing" existed around that pool of early fundamental particles plasma that existed at Big Bang. How can expansion of space time happen through nothingness? What exactly is an empty space?

I'm not asking about empty space as in vacuum (as I understand space vaccum do concur to Heisenberg's uncertain) but what is absolute nothing?