I’m quite impressed by “Cosmology in 10 Minutes” by Danielle Fong, her attempt to explain why scientists believe what they do about how this all came to be:
Guth said, suppose you started with pretty much any initial universe. Suppose you also had an extremely strong, extremely smooth field of energy. If this field started dumping energy into the rest of the universe, it would also, evenly expand space itself. The universe would undergo a period of exponential expansion — inflation — having the effect of flattening and smoothing the rest of the universe. Inflation is God’s clothing iron.
Go, read, understand.