Firing particles at gold foil revealed that atoms are almost entirely empty space

Ernest Rutherford expected alpha particles fired at thin gold foil to pass straight through, as the reigning 'plum pudding' model of the atom predicted. Most did — but a tiny fraction bounced straight back, which Rutherford compared to firing a shell at tissue paper and having it rebound and hit you. The only explanation was a dense, positively charged nucleus at the atom's centre.

— Ernest Rutherford, The Scattering of α and β Particles by Matter and the Structure of the Atom — Philosophical Magazine, 1911, building on the 1909 Geiger–Marsden experiment