The main theory at the time of Rutherford's gold foil experiment (early 1900s) about what an atom looked like was the plum pudding model. This model, proposed by J.J. Thomson, envisioned the atom as a sphere of positive charge with negatively charged electrons embedded within it, like plums in a pudding.

This model was based on the following discoveries:

* Discovery of the electron: Thomson's own work had demonstrated the existence of negatively charged particles (electrons) within atoms.

* Atom's neutrality: Atoms are electrically neutral, implying the presence of both positive and negative charges.

However, the plum pudding model had some shortcomings:

* Lack of a defined nucleus: It didn't explain why atoms were so stable, as the positive charge was distributed uniformly throughout the atom.

* Scattering of alpha particles: Rutherford's experiment showed that some alpha particles were deflected at large angles, which the plum pudding model couldn't explain.

Rutherford's experiment ultimately disproved the plum pudding model and led to the development of the nuclear model of the atom. This model proposed a dense, positively charged nucleus at the center of the atom, with electrons orbiting around it.