Atomic nucleus
Neutrons have no electric charge and protons are positively charged. Because the nucleus is only made up of protons and neutrons it is positively charged. Things that have the same charge repel each other: this repulsion is part of what is called electromagnetic force. Unless there was something else holding the nucleus together it could not exist because the protons would push away from each other. The nucleus is actually held together by another force known as the strong nuclear force.