I have heard theories for the big bang like: of another universe existing before the Big Bang, or an unstable white hole that blew up, or our universe detaching from our mother universe, or our universe coming out of a black hole. Most of these theories don't make sense, and I want to know the most-likely-to-be-true theory for how the singularity was made.
No one knows what came before the Big Bang if, indeed, anything did. Theories include:
- The Ekpyrotic universe theory where the BB was the result of the collision of branes.
- Various oscillating universe theories where the expansion of the universe ultimately reverses into a Big Crunch
- Quantum fluctuation models where a zero-net-energy universe is a quantum fluctuation from nothing.
- Various multiverse models where the universe we see is only an infinitesimal bubble in a much, much, much larger universe.
- The Hartle-Hawking idea where spacetime gets a bit scrambled and there is no before. (Really: time turns into a space dimension if you go far enough back, and there is no time.)
- Stringy stuff. Very difficult math.
I've only scratched the surface -- making models like this is an avocation to some physicists, while others have discovered that it sells books and generates publicity. All of the theories have one thing in common: There is no experimental evidence for any of them. Additionally, many of the theories have been shown to contradict currently-understood physical law. (Especially the 2nd Law of Thermodynamics.) The authors generally just shrug and write another paper.
(On the religious side, speculation about what happened before Creation has been around as far back as written history, with roughly equal success. In the 4th century St. Augustine, after being pushed to answer that question retorted that before the Creation, God was creating a Hell for people who ask such questions. That didn't stop the speculation either.)