How preposterous? Please explain.
Gawd knows, I'm not an astrophysicist, but I did read "A Brief History of Time" on a beach in Sri Lanka and understood about 1% of it. So take all of the following with a grain of salt.
AFAICS, their contention is that before time began, there was nothing. Nothing at all. No time, no matter, no space, just nothing.
Then, from a single point in space (although there wasn't any space yet) precisely 13.8 billion years ago, the universe simply appeared, instantaneously.
How do they expect us to swallow that?
If it did suddenly appear from a single point, where is that point? They say that the universe is expanding. From where? From that magical point in space that didn't exist before the big bang? Shouldn't we be able to point to it and say "That's where it all began?" Apparently, they can't.
Worse, they tell us that it' expanding in size and even more worse, the rate of expansion is increasing. It's accelerating. Where's the energy coming from to propel that acceleration?
To balance the physics books, they expect us to believe that it's accelerated by "dark energy".
And if that's not enough, they tell us that they can't account for the majority of the mass that must be present for their explanations to all work out, so they invent "dark matter".
Really?
It appears to me to be a house of cards. Hopefully JWST will bring down that improbable construction and replace it with something even more preposterous.