Science is based on the empirical study of cause and effect. The hypothesis formed by anyone has to be testable to establish a theory. Now, if the Big Bang is the effect, its cause can be studied only before it happened. But according to the cosmological theory of the Big Bang, there was nothing before the series of events occurred. No space, not time, not even matter…NOTHING!! Then out of ‘nowhere’ appeared a high temperature, high density ball of matter smaller than an atom and ten trillion trillion times hotter than the core of the Sun. This is called the ‘initial singularity.’ The energy contained in this tiny yet infinitely dense ball of fire was 4*10^69 Joules. That is 4 followed by 69 zeroes. As it started to expand, in the first second after the Big Bang, the blueprint of the whole universe was written.
That being said, coming to your question, as the cosmological theory of the Big Bang itself suggests that the fabric of space time did not exist before the big bang, there is no way of observing the cause of the event. Because TIME WASN’T THERE! Thus through science, this fact will always be unknowable.