Most people intuitively think that the Big Bang means that the Universe came into existence as an explosion of matter and energy into empty space. Actually, the process was the expansion of matter and energy and space time itself, with existence itself expanding so as to fill what had previously been mere nothingness.
It probably should be called the big suck instead of bang, and the suck hasn’t ended. The total energy in the universe is still increasing as everything is being sucked apart into nothingness but the energy density is decreasing faster.