Nothing.
"Nothing" is the bogus imaginary physics here. That and "heat death of the universe" fit together hand in glove. Obsolete ideas borne out of early twentieth century conceptual limitations.
You go gather up a pile of nothing and show it to me and I will believe you. But be careful...if you, and those who share your belief in the big bang, are correct all that "nothing" may explode in your hands and some poor sentient byproducts of that explosion will eventually confuse you with "God".
;-)