Pauli exclusion principle | 🗖 nosplit | ↑ parent "Spin" | 102, 4, 128

Initially a phenomenological guess to explain the periodic table. Later it was apparently proven properly with the spin-statistics theorem, https://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!