We understand the physics of matter roughly up to the point where it says "Electroweak Phase Transition". Anything earlier than that is just speculation, and there are many theories for it. Some have a big bang others don't.
The Big Bang (red arrow top left) is what you get if you stick with Einstein's theory. It's the simplest extrapolation. The JWST data that everyone is talking about is at the bottom right. Doesn't tell you anything whatsoever about the Big Bang.
The JWST can tell apart dark matter from modified gravity, I talked about this here youtube.com
I also (rather coincidentally) have a video coming up on Saturday explaining why we don't know how the universe began and why we will probably never know.
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