Concerning the Big Bang, how can nothingness produce mass and energy?
We don't know. It may be a cyclic physical event or one of supernatural creation.
Science is all about OBSERVATION. We attempt to understand our universe by observing how things interact. We can only model from what we see.
So far, the “Big Bang” theory of universal generation explains many observations, but many main unresolved, and there is no guarantee that this theory itself is even the correct model.
For further discussion on what we think we know, and why, see http://science.nasa.gov/astrophysics/focus-areas/what-powered-the-big-bang/
and http://www.space.com/25126-big-bang-theory.html which includes the “multiverse” implications.
I highly recommend that everyone read Dr. Stephen Hawking's very readable books “A Brief History of Time” and “A Briefer History of Time” as well as “The Grand Design” for the best non-theological discussions of universal origins. https://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Ddigital-text&field-keywords=Stephen+Hawking