It has been observed so many times, though. That's the thing. There are a number of cases in which things travel faster than light, but it's only information that is bound by that limit. And this has been observed time and again. Galaxies racing away from it is one example. The proof that you seek is, unfortunately, beyond what can be described in Answerbank. But you're taking a concept that you appear only partly to understand and applying it wrongly. There's no other way of putting it.
Faster-than-light speeds are, anyway, easily observed. I even described a possible case earlier for you to look at. Trace the contact point of a wave with the beach in the case where the wave is heading directly towards the beach. Then in that case the speed will be effectively infinite. Which is, of course, faster than light.
Spacetime expansion is the same. Since no information is being carried, the speed is unbounded from above, and indeed in early inflation was many times faster than light. The evidence of this exists, for example, in the CMB radiation, or in the fact that the measured size of the Universe is far larger than its age times the speed of light. In itself this is proof that expansion happens faster than light.
See also
http://en.wikipedia.org/wiki/Metric_expansion_of_space