Why is universe expanding? Okay, this question may sound silly:
base on the observation
Besides an expanding universe, would there be other possibilities?
Would it be possible, say, there exists a fundamental repelling interaction among all partices, which is proportional to distance (e.g. $Ae^{ar}$, where $A$ extremely small?)
p.s. I know it is a kind of silly question, but if you downvote it, at least please tell me why. 
 A: Some galaxies are known to be expanding away from us faster than the speed of light.  This is only possible if it is the underlying universe getting bigger.  It isn't possible that these galaxies are simply moving faster than the speed of light as your argument suggests as this violates the principles of special relativity.
A: I am attempting answering my question based on one fact and one assumption (actually there are one more fact/ observation - Redshift):
The fact is:
inertial frame. which, to a good approximation, earth itself (ignore its rotation, irrelevant to our discussion) is an inertial frame.
The assumption is:
Earth is no special place.
Hence the logical consequence(only possibility) is that the universe expanding instead of there exist converging interactions proportional to distance. 
A: I hate to give a mainly-link answer, but Alan Guth's video about dark energy was very educational to me.
He's the cosmologist who's a primary author on the inflationary universe theory.
Basically, if I can summarize it, there can be a kind of material that has repulsive gravity rather than attractive gravity. In addition, its density in space does not necessarily decrease as it expands, and this is the cause of the initial big bang, after which it mostly decayed into ordinary energy-matter with attractive gravity, but there's still some around, causing the cosmic expansion to accelerate.
