First, ungrounded and speculation are the same thing. Second, isn’t the existence of God just a speculation

To get scientific The Big Bang was a moment in space time where there was no space and time. Therefore there was nothing to exist and nothing to start the universe, expect for a super condense, infinite point called a singularity with a size of 0. Then as the singularity cooled it burst into what we call the inflation period creating matter and what we now know as our universe