system
Given the following post, respond as if you're commenting on Reddit:
human
Why didn't the big bang turn into a black hole instead of an explosion?

Me and my dad were talking about when all the matter was in one place why didn't it become a black hole instead of the big bang? So we couldn't find an anwser so I thought why don't we ask Reddit? 
Oh and btw what is the density of the matter in a black hole and is that the highest density possible?
Hope you understand what I have wrote and sorry for any grammar mistakes, I'm not the best at english :)
gpt
First, you need to understand what exactly is the big bang.

The big bang is not a "big explosion of matter", as you might picture. Think about larger scales, bigger than galaxies or clusters of galaxies or everything you can picture. Matter is more or less homogeneous and you can treat it like a fluid (like a lake, for instance. from far away, it looks homogeneous, but if you get close, you'll see molecules and atoms and everything).

Whenever you have such a homogeneous fluid and you turn on gravity, you find out that it is dynamical. It will either expand or contract, depending on what is the total energy of the fluid. When we look around, we see that this is what happening: everything is expanding. So we have a good estimate for the density of this fluid around us. Keep im mind that in the effective cosmological description that we have, this "universe" has an infinite volume, so expanding and contracting are terms that are related to how you measure distances.

When you switch back time, though, it means that at some point the density will be bigger and bigger and bigger. You find out that you will undergo phase transitions (for instance, at some point in the past all the matter was radiation, and before that it was some other strange fluid with different properties).

This is the big bang: it's not a statement about what was the state of the universe when t->0, but how the universe has evolved ever since. When you try to understand what happens at t->0 you get a singularity, but that's not physical, since at this point you can no longer treat all your matter content as some homogeneous fluid. We say that the "effective theory breaks down", which literally means that you can no longer trust whatever approximation you made to derive this "big bang solution".

So what you are saying is right: if you compress a fluid such that its energy density is high enough, it will form a black-hole. And certainly there might be primordial black-holes around because at some points, the energy density might become a lot bigger through some thermal fluctuations and you might just pop a black-hole. But taking the universe as a whole, you can NEVER have the whole energy content to become larger than this cutoff, which is given by the Planck energy scale, otherwise, bam, your effective description is breaking down.