The neutron is, as suggested by its name, electrically neutral. Nevertheless, it still contains electrical charges. More specifically, it is made up of one up quark (charge: 2/3 of an electron charge e) and two down quarks (charge: -1/3 e each). In total, 2/3 + 2*(-1/3) equals exactly 0. But the neutron is not one-dimensional. It has a diameter of at least 1.7 * 10-15 meters, and when three components have to be spread out over any distance, even with an overall zero charge, some difference in charge should still be detectable. Calculations from theory say that a neutron should have an electrical dipole moment of 10-16 e*cm.
In reality, however, absolutely no dipole moment can be detected. If it does exist, it must be less than 10-25 e*cm, which is a difference of nine orders of magnitude, a huge discrepancy. One solution would be the existence of a previously only hypothetical, extremely lightweight particle, the axion. Huge quantities of these tiny particles, billions of times lighter than an electron, might then be roaming around our universe unnoticed. This is because axions normally do not interact with normal matter.