A Mystery Remains
Everything in nature seems to have its opposite, male and female, yin and yang, plus and minus, far and near. . . . But one thing remains singular, in spite of theory that contradicts it: magnetic poles always are observed in pairs, never as single poles. The single magnetic pole (the magnetic monopole) is predicted by Big Bang theory (which works amazingly well for almost all observations.)
Electricity and magnetism appear everywhere in the modern world and form the basis of most of our technology. Therefore, it would be natural; to assume that they are already fully understood and no longer pose unanswered fundamental physics questions. Indeed, for most practical purposes they are perfectly well described by the work of James Clerk Maxwell (circa 1864). Modern theory works so well that it predicts the magnetic property of the electron accurately to 10 significant figures. Nevertheless, we do not understand the question of magnetic monopoles.
Several experiments in this decade have sought the magnetic monopole (in polar rocks, Moon rocks, and seawater, in mica, all to no avail. Grand Unified Theory (super good theory, never falsified—called GUT) predicts monopoles, and they should affect the interiors of white dwarfs, neutron stars, and even the Sun.
The discovery of magnetic monopoles would have a huge effect on physics. Not only would monopoles provide the first glimpse of the new laws of nature beyond the standard model (which predicted the now well established Higgs particle) but their special properties would allow us to explore that new physics in ways not possible with other particles.
 See Arttu Rajanntie, “The Search for Magnetic Monopoles,” Physics Today (69, 10, October 2016), pp. 40-46)
 See A. Rajantie, Contemporary Physics (53, 195, 2012); j. Preskill, Annual Rev. Nucl. Part. Sci. (34, 461, 1984); Y. M. Shnir, Magnetic Monopoles. Springer (2005); K. A. Milton, Rep. Prog. Phys (69, 1637, 2006).