The experimental results that led to the conclusion that atoms contain massive central regions, now known as nuclei, were primarily from Ernest Rutherford's gold foil experiment in 1911.

Here's a breakdown of the key observations and how they led to this groundbreaking discovery:

* Experiment: Rutherford fired alpha particles (positively charged helium nuclei) at a thin gold foil.

* Expected Outcome: Based on the prevailing plum pudding model, which suggested that the positive charge of the atom was evenly distributed, Rutherford expected the alpha particles to pass straight through the foil with only minor deflections.

* Actual Outcome: Most alpha particles did pass through, but surprisingly, a small percentage were deflected at large angles, some even bouncing back towards the source.

* Interpretation: The only way to explain this unexpected scattering was to propose that the positive charge of the atom was concentrated in a tiny, massive region at the center, which Rutherford called the "nucleus."

* The Nucleus: The nucleus was responsible for the strong repulsion that deflected the positively charged alpha particles. This also implied that the atom was mostly empty space, with electrons orbiting the nucleus at a much larger distance.

Therefore, Rutherford's gold foil experiment revolutionized our understanding of the atom, proving that atoms contain a small, dense, positively charged nucleus.