It’s the topic of instantaneous creation. When a believer is asked, “If God created the Universe, how did God come to be? And if he was always there, why not save a step and say the Universe was always there?”, he may retort: “Well, you can’t explain where the Big Bang came from.” Here are some thoughts on the inequality of those accusations.
(First of all, there is an inequality in that one is summary conjecture from doctrine not present in all religions. For some, this would be enough, really.)
But the person asking often recognizes Science’s fascinating ability to reveal the Universe’s complexity and analyze it, so that we can understand more. There is no question of that ability, and believers as well as Atheists are awestruck by the wonder of this Universe in which we live. This is a revelation not present in scripture. We owe this sense of wonder to Science alone.
A believer with normal schooling would accept the arguments and evidence for the Big Bang. Even if the schooling is not there, he may – because the evidence is so compelling. But the real riddle is a psychological one: he feels who-created-God and who-created-the-big-bang are similar questions, and uses it is as an, in his mind, apt counter-accusation.
The answer is simply that the evidence takes you this far, Science has advanced this far in understanding the Universe, at the present moment. Now, here’s the nub: if he believes he knows the true origin of the Universe, he should have information enough to question this mere theory of a hot dense small beginning. But he has demonstrated that he does not, because he too is persuaded by the evidence. He accepts Scientific explanation up to a point where it becomes uncomfortable; threatens his beliefs.
And throughout history, this is how supernatural causes have had to retreat, further and further, to where they are today: to a very short time before the Big Bang. Causes of unsuitable weather, bad crops, illness, wonders of Nature, and intricate flora and fauna have all to a very large extent given way to Scientific, evidence-based explanation.
It was enough to turn a theist into a deist; now, it’s enough to turn a deist into an adeist.