All matter is mostly empty space!

The world feels very solid around us, but in reality almost all of the size of each of the atoms that make up matter is empty space. The nucleus is tiny in comparison to the size of the atom and the electrons that inhabit the orbital cloud are much smaller still. If you imagine it as a solar system with the nucleus being a tiny Sun and the electrons as mere asteroids or mere specks inhabiting the orbits as far as erstwhile planet Pluto, you can see that the majority of the atomic size is simply space!
In fact, if you could squeeze all this empty space out of all the atoms, pushing all the nuclei and electrons together, then the whole human population of Earth would fit into a volume roughly equal to a sugar cube!