You may have heard that the atoms that make up our bodies, the ground we walk on, and all the matter in the universe are essentially vacuums. If so, the question arises: why don't we fall into the ground when we walk, why can't we walk through walls like ghosts?
The simple answer is that the force that prevents us from doing so is the electromagnetic repulsion force.
The answer is simple, but it needs to be clarified.
From a quantum point of view, the idea that atoms are basically "vacuums" makes no sense. The volume of an atom is filled with the wave function of its electrons, or from the point of view of quantum mechanics, there is a local excitation of the electron field in this region of space, both of which are related to the "empty" vacuum state.
The concept of vacuum is actually quite complicated, because the concept of "space is empty when there are no particles in it" is very different from the formal concept of "vacuum is an unexcited vacuum state".
The space inside the atom is definitely not a vacuum, it is filled with electronic states. But it is likely that you will never find "empty" space, i.e. "no particles present during measurement".
The statement that the space inside the atom is "mostly empty" is not true, because the electrons are not so visibly localized, unless there is some kind of interaction (such as measurement) that actually forces them to do so. When they don't interact, their state "smears" across the atom, forming something sometimes called an electron cloud, where the electron cloud or orbital represents the probability of finding a particle at any given location.
This strangeness is one of the reasons why quantum mechanics is so fundamentally different from classical mechanics – all of a sudden, many parts of the world become completely different from what we are used to on a macroscopic level, especially our intuition of "vacuum" and so on. Let us down on a micro level.
So, when you walk, you won't fall off the ground because you are suspended in the electrostatic field. When you're sitting in a chair, you're not actually touching it. Each atom is surrounded by an electronic shell. When two atoms are close to each other, their electron shells repel each other despite the fact that the total charge of each atom is 0. This is a very useful property of nature that makes our lives so much easier.
Three minutes to talk about popular science