2.6. The Monopole Problem
There is no way the Monopole problem can be regarded as merely a feature. In 1979 Preskill  showed how quite generally a significant number of magnetic monopoles would be produced during the cosmic phase transition required by Grand Unification. Being non-relativistic ( a-3), they would rapidly come to dominate over the ordinary matter (which was then relativistic, a-4). Thus it appeared that a Universe that is nearly 100% magnetic monopoles today was a robust prediction from GUT's, and thus the idea of Grand Unification was easily ruled out.