What's in between an electron and an atom

I know electrons 'revolve' around atoms. But what's in between these two things?

When you get down small enough to the realm of electrons and atoms, the concept of "in between" begins to lose meaning. Electrons are described by a three dimensional wavefunction which tells us that it could somewhere in the viscinity of the atom, with higher probabilities to be in some regions than others. Electrons do not revolve around the "atom", they revolve around the protons and neutrons in the nucleus. The electrons, protons, and neutrons together make up an atom. Also, at the atomic scale, the uncertainty principle comes into play which guarentees that we cannot know the exact position or momentum of the electron at any moment.

as Mike says

I'm not even sure that electrons "revolve around atoms". They have spin, sure, but in quantum mechanics they don't orbit around the nucleus (orbiting charges radiate electromagnetic energy and die) but are described by a probability function that says what the chance is of finding them in a particular place.

empty space.