As Atheists, we are forced to find evidence for all things we perceive, and some that we do not. It is a rather difficult task to not just fold our hands and jump at the first conclusion given. Even if there is a question to which we currently have no answer, an Atheist must acknowledge that lack and, lacking sufficient evidence, move on until technology improves more. A perfect example of this situation is a very common question asked by Theists, and people in general.
"Where did the universe come from?"
Many Theists believe that, to be an Atheist, you must believe that the universe came from nothing, a giant explosion that created all the matter and energy in the universe. This simply is not true. To be an Atheist, you must acknowledge that nothing is able to come from nothing. EVERYTHING has to come from somewhere. This is a difficult concept to grasp, as it also means that the universe has always been, in one form or another.
The "Big Bang" that Theists point to is not a "Bang" at all. If there was an explosion of any kind it would destroy, not create. Instead of an explosion, the "Bang" was an expansion of material from a singularity. A quick one, but an expansion nonetheless. The term "Big Bang" was created by those who did not believe in an expanding universe, though it has been proven mathematically, and kinda stuck. So, no explosion of or from nothingness.
As for what a singularity is, that's a bit harder to explain. Anyone who knows what an atom is knows that is is more than just the material that makes an atom, there is also a lot of space between the electrons and everything else. If you were to remove that space, everything could be made MUCH smaller. Also factoring in that matter can become energy and energy can become matter, things can be compacted even more. And this singularity isn't just a ball of energy floating around, it also has every force of nature floating around inside like a miniature, energy universe without any room. This is what became the universe, forcing everything to expand away from the center and from everything else.
So, where did the singularity come from? No one knows that one just yet. However, that is not to say we will never know. Science is always advancing and finding ways of looking farther and farther back by examining the present. The singularity had to come from somewhere, and that had to come from somewhere, and THAT had to come from somewhere. There will always be something before and something after what we know.
Everything had to come from something, and nothing can ever be completely destroyed. It all goes somewhere.