to rephrase my Q, is it only on account of empirical evidence that we accept light and EM waves travel through vacuum or is the actual propagation mechanism understood.

to give an analogy, it is my understanding that, it is only on account of empirical evidence that we accept that the speed of light is a constant and not because we can explain why it has to be 3x10[sup]8[/sup] m/sec.

is the propagation of EM waves in vacuum/no media similarly on account of empirical evidence?