The force that usually stops two nuclei getting close together is the electrostatic repulsion between their positively charged protons.

Here's why:

* Atomic Structure: The nucleus of an atom contains protons, which have a positive charge.

* Like Charges Repel: Positively charged particles repel each other.

* Short Range: The electrostatic force is very strong at short distances.

When two nuclei approach each other, their positively charged protons experience a strong repulsive force that tries to push them apart. This force is the main reason why nuclei don't easily fuse together.

Overcoming the Repulsion:

In order to overcome the electrostatic repulsion and allow nuclei to fuse, extremely high temperatures and pressures are required. This is what happens in stars and nuclear weapons, where the energy of the particles is high enough to overcome the repulsive force.

Note: While electrostatic repulsion is the primary force preventing nuclei from getting close, there are other forces involved, such as the strong nuclear force, which is attractive at very short distances and helps hold nuclei together.