What seems incomprehensible about that? Of course, there is nothing even remotely similar in our personal experiences, but we grasp things all the time that we haven't gone through ourselves.
In roughest terms, imagine a grenade exploding, with its contents radiating out in an expanding sphere.
So, what's troubling? Its improbability? Its immensity? How all space and matter could begin in an infinitesimally small volume? The fact that a seemingly highly-ordered universe is its result?
This is it. I don't understand the physics.
In my small mind, the 'Big Bang' was either:
1. a 'leak' from another dimension
2. the finger of God
“So, what’s troubling?”
Where did it come from?