What physical force briefly resists further collapse in a remnant forming a neutron star?

Neutron degeneracy pressure

When the collapsing stellar core retains a mass above approximately 1.4 solar masses but below about 3 solar masses, a powerful force manages to briefly halt the catastrophic inward crush of gravity, resulting in a neutron star. This force is known as neutron degeneracy pressure. This pressure arises from the quantum mechanical principle that dictates that neutrons, being fermions, cannot occupy the same quantum state. As matter is squeezed to extreme densities, the neutrons resist further compression by exerting an outward pressure. While this pressure is incredibly potent—sufficient to maintain an object where a teaspoon weighs billions of tons—it is ultimately insufficient if the core mass exceeds the higher threshold, allowing gravity to win out and form a black hole.