The origin of the singularity that gave rise to the universe is a matter of speculation; nothing is really known yet about it. You are mistaken in saying that something cannot be created out of nothing. Virtual particles appear out of the vacuum, linger a moment, and then annihilate and disappear, and this is happening continuously on an unimaginable scale all around us all the time. Our own universe may have originated as such a quantum fluctuation in a previous universe.
There is no such thing as an "atheist theory." Theories are proposed by scientists, and whether they are atheists or not has nothing to do with it. Scientists follow evidence wherever it leads.