Opposite sides of the sky agree to one part in a hundred thousand, and under standard expansion they were never in contact. The cosmic microwave background's temperature is uniform across regions separated by more than about two degrees on the sky, regions whose past light cones, traced back through ordinary expansion to recombination, never overlap: no signal, thermal or otherwise, could have equilibrated them. This is the horizon problem, and it is not a measurement anomaly but a bookkeeping fact of the standard expansion history: the CMB sky comprises tens of thousands of causally disconnected patches that nonetheless share a temperature to exquisite precision, as if a conversation had happened that the model's causal structure forbids.
Inflation answers by rewriting the early timeline: a phase of accelerated expansion takes a single pre-equilibrated micro-patch and stretches it beyond the observable universe, so today's disconnected regions share history after all. The solution works, at the price catalogued across this series: an inflaton field never identified, a potential tuned by hand, initial conditions (a smooth inflatable patch) that restate the problem at smaller scale, the trans-Planckian pathology in the mode accounting, and a graceful-exit and measure problem attached to ending the episode. Inflation explains the uniformity by positing exactly the unobserved machinery needed to explain it, which is why its absence of confirmed signatures (no primordial B-modes at any sensitivity yet reached) keeps the rental agreement uncomfortable.
The standing is the original motivating puzzle of modern early-universe theory: solved-by-subscription within the standard model, dissolved entirely in any framework whose origin event thermalizes the whole volume at once.