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#171
The fundamental concept is that space and time are one and the same thing, which is often why its referred to as spacetime. Its a physical entity comprsing of the basic forces of nature, gravity, eletromagnetic forces, nuclear forces and dark energy/matter. Galaxies are material objects made of the same elements that you and I are.
Universes that are moving apart from one another display red-shifting of the electromagnetic spectrum. Since the universes are located in different regions of spacetime, then space itself must be inflating - see the example above demonstrated with the dots on the baloon : the dots represent the universes, and the rubber skin of the baloon represents spacetime. If universes/galaxies move away from one another, then its because the space they are in are also moving away from one another.
Sound waves travel at considerably slower speeds than light, which is why the example is easily illustrated using a stationary observer and a passing car. If you were travelling along the side of the road at a speed only slightly lower than the passing car, then the Doppler effect is not as strong. Of course, if you were travelling faster than the car you would never experience the Doppler effect : remember its about your position relative to the observation.
If you are after some bedtime reading, I recommend:
Metric expansion of space - Wikipedia, the free encyclopedia