Not mostly empty space, entirely empty space. All fundamental particles are point like and have no physical size.

Except ..... that's not the right way to measure size, you measure the size of a particle by the forces interacting with it.

By that measure an electron has an infinite size (since an electric field is infinite), and a chair has exactly the size it appears to have, because that's the size measured when you interact with it electromagnetically.

Objects don't have one single size, their size varies depending on what tool you use to measure them.

What's the "size" of pressure and vibration?

All parts, remain as parts of the environment, the whole.

Neutron stars are one of the few exceptions.

You don't actually know that. Neutrons are made of quarks, which are probably point like, i.e. the inside of a neutron is entirely empty space.

And if they're not, then eventually they're going to be made of something (that is going to be made of something and so on) that is pointlike. Very probably. If not, which will be a surprising result, then it's still going to be empty space.

Yeah, but they push the pauli exclusion principle to the extreme, meaning there's hardly anywhere within its structure for particles to not exist.

Oh right! The universe well and truly is an incredible place.