in fairness, "big bang" is kind of a misnomer, it was a term originally used to make fun of the concept of an original expansion. it isn't thought to be an "explosion" at all
but we know the universe is expanding. we know the expansion is accelerating (though to my understanding, don't know what is driving the acceleration... hence the term 'dark energy'). given that, they've calculated backwards to a starting point.
there is the Futurama-esque idea that there is an ever-going oscillation of the universe... an expansion followed by contraction, etc.
all we can confidently know at this point is that the universe has expanded from a singularity