re: The explanation.
Imagine that an uncounted number of photons are created in a single spot, in a universe that has nothing else in it except the light that was just created. As the light moves away in all directions from that spot, it leaves behind dark, since the dark advances in all directions from the point where it was created, (point of origin) it must expand in volume with the cube of light speed, therefore its linear speed (measured from two photons moving in opposite directions) is twice the speed of light.
The question then remains that if there is nothing in front of the advancing light, such as other photons, would it still move at the measured speed?