After the Big Bang, what is the next most popular theory on the formation of the universe? What are some of the flaws in this theory? Why is the Big Bang the more accepted theory?
Other than God did it? None.
Not to oversimplify this but the very nature of "theory" means that an absolute proof does not exist. In the case of the development of the early universe, it will most likely always remain in the theory domain.
The big bang theory simply does the best job of explaining the creation of the universe from its very beginning. All the various explanations of what happened and how it happened are in keeping with the known laws of physics.
A better theory of how the universe got its start and why we are where we are now may arise but until that happens, the big bang theory is simply the best we have.