it depends on what you define to be a universe. scientists refer to it as this discrete space that we, along with all observable things, occupy. There may be universes beyond our universe, but mass and energy cannot transfer among them so they cannot be observed, and there is little sense in describing two separate universes as if they are open and simply connected, because then they would be observed to belong to each other as one universe. This makes it difficult for me to believe that there are tiny universes within this one. If every carbon atom was a tiny universe, what happens to those carbon universes when I'm cooking chicken on the grill? Energy in this universe is conserved, but in the tiny carbon universe a stupendous amount of heat is coming in from beyond the boundary, which implies an open and simply-connected region (ie, energy in the tiny universe is not conserved unless it shares energy from our macro universe).