Imagine a sugar cube that weighs as much as Mount Everest. That is the density of a neutron star. It violates our intuitive understanding of the physical world.
The Collapse
When a massive star dies, gravity wins. It crushes protons and electrons together until they merge into neutrons. The space between atoms—which is 99.9% of matter—is squeezed out.
The Result
What's left is a city-sized sphere containing the mass of our sun. If you were to drop a marshmallow on its surface, it would hit with the energy of a nuclear bomb. It is the most extreme environment in the universe outside of a black hole.