 Pauli exclusion principle

Initially a phenomenological guess to explain the periodic table. Later it was apparently proven properly with the spin-statistics theorem, physics.stackexchange.com/questions/360140/theoretical-proof-of-paulis-exclusion-principle.

And it was understood more and more that basically this is what prevents solids from collapsing into a single nucleus, not electrical repulsion: electron degeneracy pressure!

Bibliography:

The Pauli exclusion principle is a fundamental principle in quantum mechanics, formulated by physicist Wolfgang Pauli in 1925. It states that no two fermions (particles with half-integer spin, such as electrons, protons, and neutrons) can occupy the same quantum state within a quantum system simultaneously. In the context of atomic structure, this principle explains why electrons in an atom fill available energy levels in a specific way.