Hi Johnnie,
This is an interesting question! :)
When you press something indefinitely with the help indestructible stuff, you will eventually start the process of creating a singularity a.k.a the black hole. So to begin with, let’s consider you are compressing an object like an iron ball for example. When you increase the pressure in the container, the molecules in the ball start compressing and in the process the temperature rises (the molecular kinetic energy increases). The more you compress the more the heat is added (as work done + rise in the average kinetic energy of molecules). This will eventually lead to a change in the phase of the iron ball.
Take a look at Iron’s phase diagram below.
Let’s say you have reached a temperature of 3000 degree Celsius and pressure beyond 500 bar.
At this point, iron would only exist as a hot molten liquid. When you keep increasing the pressure with the ‘indestructible and non-reactive’ piston of yours, the molecules will soon rip itself apart and soon the electrons will rip itself from the atoms (as they will reach very high energy states), forming a very hot plasma.
You keep increasing the pressure, you would have added enough energy to overcome the repulsion between the electrons and nucleons (a countering pressure called degeneracy pressure). This is the point where you will cross the electron degeneracy pressure.
After overcoming that, you will eventually overcome the proton degeneracy pressure. And as the energy of electrons and protons are now increased, electron capture begins and all the electrons and protons will form neutrons, leaving a soup of neutrons in your unbreakable container.
Now your unbreakable container is a neutron star!
When you increase the pressure, you will overcome the neutron degenerate matter and might create a quark star (where the neutrons will now only be existing as their constituent particles as the up and down quarks). Now, you will have to increase your pressure to overcome the pressure between these quarks (quark degeneracy pressure).
And when you overcome this, you will be able to confine all of the matter in that indestructible container into a point, creating the singularity.
