We do not know practically anything about what was before the ‘big bang’. Theists can believe there was a Creator but that is a matter of faith. Those who do not believe in a creator need to invent hypotheses that might explain what happened, or be satisfied with not knowing. Scientists are usually not satisfied with not being able to explain.
There is a common problem in the ‘semi-scientific’ hypotheses that attempt to explain what was before the big bang. The problem is that the calculations assume that everything works according to the laws and phenomena we observe in this universe. For example, if we observe something happening in the vacuum of space, we can use that as a starting point because ‘vacuum is nothing’, like there was nothing before the big bang. That kind of assumption is simply false.
Vacuum is a location in the space-time of this universe, including very few particles but affected by nearby force fields and with photons and other forms of energy passing through the ‘empty’ space. That is very far from ‘nothing’. If there is no space/locations, no time, no force fields, no laws of this universe operating, what can we use as the starting point or basic assumptions on which to build?
Multiuniverse, including many parallel universes, is a hypothesis that can be compared to the simulation hypothesis. There is practically no convincing evidence for these hypotheses. Even if the hypotheses would be true, they would not explain the beginning of everything, only push the starting point further into the distant past.
That is also the weak point of the cyclical universe hypotheses.
The parallel universes are a consequence of the multiverse hypothesis. One purpose of the multiverse hypothesis was to explain the fine tuning problem - why are all the parameters of this universe tuned to be exactly correct for the known life?
If our conditions are a random fluke, there should have been an enormous number of universes that had ‘less lucky’ conditions. If there was a ‘generator’ that gave birth to a huge number of universes, the other universes could exist as ‘parallel’ universes. What the advocates of such parallel universes tend to forget is that the probablility that a parallel universe has conditions suitable for life is really, really tiny.
Some parallel universe hypotheses start from the assumption that free will leads to ‘branching’ of potential universes. Every time you make a choice, your choice might lead to a future that looks different. It is purely imagination to speculate that all the alternatives lead to and exist as ‘parallel’ universes. There is no support for the dreamlike imagination that such parallel universes ‘pop up’ at all the moments of the free choices and that these alternative futures exist in somewhere else than the creative mind of a being.
At this point of knowledge, we simply have no convincing evidence to support any of the ‘semi-scientific’ hypotheses. We do not either have such (scientific) evidence for the existence of the Creator. Personally, I see the ‘Creator hypothesis’ as the best of all the hypotheses that exist but that is a matter of faith.