Why did the Big Bang explode? We assume that the laws of physics moments after the Big Bang are the same as the laws of physics now.
The volume of the universe was very small, and the density very high, certainly high enough to undergo gravitational collapse. Why didn't it?
Physicists attempt to recreate the conditions of the early universe in particle accelerators, however it useful to consider the ultimate atom smasher, one capable of creating arbitrarily high densities and temperatures: a black hole. In the process of collapsing to singularity, mass must pass through conditions similar to the early universe, including whatever it was that caused inflation. Why aren't black holes exploding like the big bang?
There must be another repulsive force, preventing a black hole from collapsing to singularity, or even to early universe densities. Its works only at extremely small distances, distances not encountered in normal matter densities away from black holes.