If the cosmic microwave background were perfectly smooth, what would remain unaccounted for?

The correct answer highlights a fundamental aspect of cosmic structure formation. If the cosmic microwave background (CMB) were perfectly smooth, it would imply that the early universe had no density fluctuations. The CMB represents the afterglow of the Big Bang and shows slight temperature variations that reflect these density fluctuations. These variations are crucial because they are the seeds from which all cosmic structures, including galaxies, formed.

In a scenario where the CMB is perfectly smooth, there would be no mechanism to account for the eventual clumping of matter necessary for galaxies to form. Without these fluctuations, all matter would be uniformly distributed, preventing the gravitational interactions that lead to the formation of galaxies over time. This directly connects the observed inhomogeneity in the CMB with the large-scale structure of the universe.

The other choices relate to different concepts in cosmology, but they do not directly connect to the smoothness of the CMB in the same way. Dark matter, primordial stars, and the expansion of the universe are significant aspects of cosmic evolution, but they do not hinge on the presence of density fluctuations in the CMB as firmly as the formation of galaxies does.