Another problem with the Big Bang since it originally started from a singularity is that it originally was a black hole. And nothing can escape a black hole.
The singularities that appear inside black holes pose a big problem for physicists. Crossing a black hole’s event horizon is like jumping into a river upstream of a waterfall, at a place where the water flows faster than you can swim. Whatever you do, you’ll end up being pulled down the waterfall. Likewise, whatever falls into a black hole is pulled down into the singularity. And once there, it reaches its end.
The Big Bang is a singularity, too. If you run the expansion of the universe which we observe today backwards , then the density of matter must have been larger the younger the universe was, all the way back to an initial moment where the density must have been infinitely high: it must have been singular.
Black holes are regions of spacetime from which nothing, not even light, can escape. A typical black hole is the result of the gravitational force becoming so strong that one would have to travel faster than light to escape its pull. Such black holes generically contain a spacetime singularity at their center; thus we cannot fully understand a black hole without also understanding the nature of singularities.
We do not observe any black holes in the universe to “uncollapse”. Things can enter a black hole, but nothing can escape it. So, what would cause the mother of all black holes, the Big Bang singularity, to uncollapse and allow all the matter of the universe contained in it to escape? Another mystery for cosmologists.