In the beginning there was the original Big Bang model, that explained the expansion of the universe using general relativity ( which described successfully gravity at the scale of the universe), and the known particle physics interactions.
But then, when the cosmic wave background radiation was measured, it showed a uniformity in any direction at the level of $10^{-5}$ .
This is also the horizon problem and it is inextricably tied with the fact that General Relativity is the theory of the Big Bang , with special relativity for flat spaces.
The horizon problem (also known as the homogeneity problem) is a cosmological fine-tuning problem within the Big Bang model of the universe. It arises due to the difficulty in explaining the observed homogeneity of causally disconnected regions of space in the absence of a mechanism that sets the same initial conditions everywhere
For a general uniformity in temperature to exist in the universe at the time of the photon decoupling at 380.000 years after the Big Bang the particles in the various regions of space should be able to interact and come to a thermodynamic equilibrium. This cannot happen at the time before the photon decoupling because of relativity, there are regions of the universe which do not interact with each other, due to the light cone geometry, so the uniformity is not explainable with thermodynamics.
The theory of inflation by introducing an effective quantum mechanical theory, allows for the uniformity observed, as it is explained in the link.

This is a model that explains observations, though there is not as yet a definitive quantization of gravity.