It's not complicated. The universe is not flat in regard to the measuring of the rate of time or of distance. Clocks in different places move at different rates of time. The detection of the collision of two neutron stars by way of gravitational interferometers has verified the existence of gravitational waves which is the dilation or variation of distance. This means that both parameters to measure the speed of light are not constant.This book will investigate what the actual effects are.
The speed of light is always constant mathematically. The problem is that the math is restricted to a given frame of reference with a given rate of time and a given measure of distance. Other frames of reference have different rates of time and different measures of distance. This book will address exactly what this means.
The speed of light also effects the observations designated as dark matter and dark energy. This book will also address these topics.