In order to understand the nature of matter one has to study the atomic nuclei which contain practically all of its mass.

Nuclear constituents (the nucleons) - the proton, the neutron (about same mass as proton but doesn't carry an electrical charge -- therefore the nuclear force which keeps these particles so tightly bound cannot be of electromagnetic origin)

The nucleus is about 100,000 times smaller than the whole atom. Its matter must be extremely dense. Nucleons respond to their confinement with high velocities, about 40,000 miles/second.

The essential new aspect of nuclear matter which accounts for its unusual properties is the strong nuclear force. It acts only when the nucleons come very close together, then the force is highly attractive. When the distance lessens the force becomes strongly repulsive. The nuclear force keeps the nucleus in an extremely stable, though extremely dynamic equilibrium.