This is the nucleus . To give you an idea of scale. If the atom were the size of a large room, its nucleus would be the size of a pinhead. The electrons round the nucleus would be like mosquitoes buzzing round the room. The rest would be empty space.
Solids exist because atoms attract one another and cling together.
If one atom tries to get too close to another, however, attraction turns to increasingly strong repulsion. This repulsion is so strong that it is almost impossible to squash liquids or solids into a smaller volume. However, a metal ball can be squashed to about half its size for an instant, using specially shaped charges of high explosive. This is what happens to the sphere of plutonium at the centre of a nuclear bomb . In stars that have collapsed at the end of their lives the pressures can be so high that the atoms collapse completely.