Final answer:
The Inflationary Hypothesis suggests that the uniform temperature of the CMB is due to regions of space being in close contact before a rapid expansion stretched them apart, thus explaining why regions not in causal contact now appear to be at the same temperature.
Step-by-step explanation:
If different regions of space were never in contact, the question of why they are exactly the same temperature pertains to the uniformity of the Cosmic Microwave Background (CMB). This uniformity, seen as regions having precisely the same temperature to about 1 part in 100,000, is paradoxical under the standard Big Bang model, which posits that these regions were never in causal contact and should not have been able to exchange information or energy to equilibrate temperatures.
The Inflationary Hypothesis explains this remarkable sameness. It postulates a rapid expansion of space before the usual expansion described by the Big Bang model. During this inflation, regions that are now far apart were once close together and thus were able to reach the same temperature before being stretched apart across the cosmos. This would mean the uniform temperature of the CMB is a remnant of that initially uniform, dense phase of the universe.
Furthermore, the tiny variations in the CMB temperature that we do observe are consistent with small fluctuations in the density of matter at the time of decoupling. These slight density differences are thought to be the seeds of the large-scale structures we see in the universe today, like galaxies and clusters of galaxies.