OK...here is a question I have had for a long time, but no athiest has ever given me a good answer.
My first response is, why Atheists? Do you assume one must be an Atheist in order to accept the natural process of biological evolution?
If it takes matter to make matter where did all the matter come from?
Matter comes from energy. Energy comes from matter.
Something had to create the matter at one point in time. Can someone explain to me where evolution explains where matter first started (even before the big bang)? Again, even if it was a small dot it was still there...where did it come from?
Biological Evolution has nothing to do with the "Big Bang", or the singularity, or the origins of matter/energy, or even the origin of life itself (abiogenesis).
Now, what you are really asking is not a biology question. It's a physics question.
Nor does it have anything to do with "atheism".
The "Big Bang" is a misnomer.
It was the initial rapid expansion of the singularity.
All potential for space/time and matter/energy was contained within the singularity.
The expansion continues to this day in our rapidly expanding Universe.
Where did the singularity/universe come from?
That is one of the hardest questions of all.
All physical laws of causation are within the singularity/universe.
So it cannot be said that any cause is necessary.
The singularity/universe could have simply began to exist. Much like we see matter "pop" in and out of existence at the quantum level.
There may have been a "First Cause". But, again, it cannot be shown to be necessary.