Final answer:
The helium nucleus is held together by the strong nuclear force, which overcomes the electrostatic repulsion between the positively charged protons.
Step-by-step explanation:
The main force that holds a helium nucleus, consisting of two protons and two neutrons, together is the strong nuclear force. Unlike the electrostatic force which causes two protons to repel each other due to their positive charges, the strong nuclear force works over very small distances within the nucleus to hold the protons and neutrons together. This force is much stronger than the electrostatic repulsion between protons and it acts not only between protons but also between neutrons, and between protons and neutrons.