When I think of an explosion it lasts a fraction of second. When the entire mass of the universe explodes, how much time passes before the perfect sphere slows down, from traveling at/near the speed of light from the forming of atoms? Or is the big bang still happening, but we are inside the sphere, while the shock wave still moves through space?
The answer to your first question can be answered regardless of your misunderstanding of Big Bang. The time passing before atoms begin to form is 379,000 years, which is the time it took for the temperature to drop sufficiently for atoms not to be constantly ionized.