there was no only one explosion but many of all sizes, big and small, and it is still going on, as different systems form and collapse, however the total sum of energy remains the same. in order for the Big Bang to happen the energy must be already there, it only changes its state and can go big boom in some arrangements, that is not to say that the boom happened out of nothing. nothing begets nothing, and it is in dichotomy as the opposite of everything, which is the total energy in the universe.
"In physics, the law of conservation of energy states that the total energy of an isolated system cannot change—it is said to be conserved over time. Energy can be neither created nor destroyed, but can change form, for instance chemical energy can be converted to kinetic energy in the explosion of a stick of dynamite."
http://en.wikipedia.org/wiki/Conservation_of_energy

the universe is the trope, i.e. the energy changes via different states of matter.

"The only thing that is certain about mathematics is that it does not describe the universe or anything fundamental.

Sir Fred Hoyle never accepted the Big Bang - even though he named it. I'd like to have seen his reasoning - he died 2001."

(via G+ Ian Beveridge)

my comments to post:
https://medium.com/the-physics-arxiv-blog/ed7ed0f304a3