I’ve been doing a little big bang thinking. If the big bang had a point source then you’d think the expansion would be spherical.
Although there is more than one shape suggested for what resulted, maybe it threw out it’s contents in a shell like a nova. Mabey it’s just a big ball.
Our OBSERVABLE universe is spherical. It has a radius and supposedly a boarder. What if the universe we see is contained withing a shell? Don’t know if it’s worth thinking about further.