If there was nothing before the Big Bang occurred, then in theory there was nothing to cause it to expande/explode. How do scientists explain this phenomenon?
Presently there is no agreed upon answer.
Previously the generally accepted answer were the oscillating universe theories. The idea was that the big bang would be followed by a big crush. The universe would expand for a time and then contract forming another big band. This allowed for the concept of material realism that matter and energy were basically eternal and self existent.
Then in 1997 and 1998 studies of supernovas provided evidence that the rate of the expansion of the universe was increasing. This empirical data indicates that the contraction of the universe needed for the oscillating universe theories will never happen.
The implication of the supernova studies is that the universe is an open system there must be something that exists outside of the space, time, matter, energy continuum of our present universe.
What exists outside of our universe is beyond scientific observation or experimentation. One theory is the multi universe theory. A variation of the multi universe theory is the white hole black hole theory and the idea of worm holes. Chaos theory or nothing really isn;t nothing theory has also been proposed.
Scientists are reluctant to give up the concepts that matter and energy are all that exists because matter and energy are all that science can investigate. That there is something non material outside of the universe is regarded as unscientific. Hence the different theories to explain where the present universe could have come from.