I thought the big bang theory was falling out of favor because of the new evidence that the physical position and movement of some heavily bodies contradict a model with all matter generated from a midpoint. Eg, we have discovered heavenly bodies that are aligned in long walls among themselves that defy an alignment and motion that suggests they came from one centerpoint.
I thought that is why the big bang theory is becoming outdated. I though the scholars now can’t support the theory that all matter was propelled from one centerpoint radially, uniformly in all directions, as the origional big-bang theory would suggest.
The big bang theory was never a theory to try and explain everything. It was proposed as a result of the observation of the expanding universe. It could never explain what happened before the big band. The multiple universe theory is a higher level concept that says That the big bang and expansion of the universe could have been caused by two separate universes colliding. It could also explain why gravity is as weak as it is in ours. I did see an interesting documentary about a group of scientists who found a quasar in an arm of a galaxy that appears to have a very different red shift than the galaxy it is attached to, throwing into question the expanding universe model and thus the big bang theory. They were saying that new galaxies were born out of the center of older ones. I am sorry but I don’t remember the title of the show. They put forward some compelling evidence though.
Of course the test of the validity of a theory is whether it corresponds to reality and it has to be falsifiable. I don’t know how scientists could detect other universes but I believe anything that exists can be discovered and proven by science eventually.