Rutherford discovered that atoms can have structure by creating a small compact nucleus, surrounded mostly by empty space. They fired particles at them. The particles could be deflected slightly by atoms, but that’s about it. Rutherford discovered that the majority of particles passed through these molecules almost unaffected. However, some bounced back in the same direction they came from. This was quite surprising when compared to the way a rifle bullet would bounce back from a sheet tissue paper. It made sense when the model was redesigned: although the compact atomic nucleus proved difficult to hit, the particle bounced back from their original locations.
What does an atomic nucleus actually look like? What is the internal structure of an atomic nucleus? You can repeat the experiment at higher energies. We discovered this by studying the way that nuclei bounce back incoming particle, and how atomic nuclei themselves are composed of smaller particles (protons, neutrons, and lots of empty space).
What does it look like for protons and neutrons? Again, this experiment can be done at higher energies and we eventually discover that they have an internal structure. They are not fuzzy balls but instead of being small particles with less or more empty space between them.
However, this type of experiment with electrons shows that there is no internal structure. The electron is smaller at higher energies, which means that it can be localized at a better point. However, no internal structure is revealed. The electron is a point particle, which is uncomposed of any other particles. It has no structure.
This information, derived from experiments, is reflected within the Standard Model for particle physics. In which there are many “fundamental” quantum fields. Each type of known particle is represented by a field. Individual particles are excitations of these areas. The electron field is one of the fundamental fields.
The model will need to be changed if higher energy experiments prove that the electron does indeed have an internal structure. The model is currently based on our experimental evidence that the electron exists as a fundamental particle.