Final answer:
The strong nuclear force is the universal force that acts exclusively on protons and neutrons within a nucleus and is responsible for holding the nucleus together.
Step-by-step explanation:
The universal force that acts only on the protons and neutrons within a nucleus is the strong nuclear force. This force is responsible for binding protons and neutrons together in atomic nuclei, and it acts only over very short distances of about 10-15 meters. Unlike the electromagnetic force, which can be both attractive and repulsive and acts over infinite ranges, the strong nuclear force is purely attractive and much stronger, but with a very limited range. It is also distinct from the weak nuclear force, which, while it also acts at short ranges, is significantly weaker and responsible for certain types of particle decay.