What experiments, other than Hubble Expansion, support the Dark Energy theory? Dark energy is introduced as a constant inside Einstein's equations. Its primary purpose, from what I understand, is to make Einstein's equations compatible with the accelerating expansion of the universe. As a consequence, of the "predictions" of dark energy is the expansion of the universe according to Hubble's law. 
I know there are numerous experiments that verify this expansion (and its acceleration), and thus indirectly support the dark energy theory. My question is:
Are there other factors that give credit to the existence of dark energy?
Are there any experiments that support this theory, but not only through the verification of Hubble expansion?
 A: I think the strongest evidence comes from the CMB fluctuations, namely the location of the first acoustic peak. This gives the overall geometry of the Universe ($\Omega_{tot}=1$; the Universe is flat). Then with a multitude of observations of dark matter (e.g., galaxy cluster counts, large-scale structure, and weak lensing) to get $\Omega_{matter}=0.3$, we are left with
$$\Omega_{\Lambda}=\Omega_{tot}-\Omega_{matter}=0.7$$
A: No, there isn't much beyond the acceleration parameter of the universe to support DE.  In fact, if you're willing to abandon homogenity and isotropy, you can even get away without DE by choosing a void model, where you replace a fine tuning of the matter distribution with a fine tuning of the dependence of the density on radius from the 'center of the universe'.  
A: It is true that nearly all of the observational evidence for the accelerating universe - that is, the presence of dark energy - comes from the measurements of the expansion rate (the Hubble parameter H(z), for aficionados). These measurements are usually not direct measurements of the expansion rate, but rather measurements of distances (to type Ia supernovae, for example), preferred length scales in the distribution of galaxies or in the distribution of cold and hot spots of the cosmic microwave background, and other geometrical measurements. All of these things directly depend on the expansion rate, and they have been the fundamental means of how we found out about dark energy. 
A qualitatively different signature of dark energy is its effect on the growth of density fluctuations in the universe - that is, the rate at which galaxies and other objects form in time. Recent measurements (of galaxy distribution at different cosmic epochs etc) indicate that measurements of this growth also indicates dark energy. [In fact, ALL of the measurements in cosmology are in perfect accord with the presence of dark energy.] However, in General Relativity, the growth rate can be shown to ALSO depend mainly on the expansion rate H(z). 
Therefore, the answer to Bruce Connor's excellent question is unfortunately not clear-cut: while many distinct experiments indicate dark energy, essentially all of them are based off of measurements of the rate of expansion - or the Hubble parameter H(z). 
A: Space expansion? yes. It is measured in relation to the atoms around us.
What if the atom can vary its dimensions thru time?
I'm pretty sure that no one presented evidence that the atom is invariant.
And yet, everybody is claiming that the universe expands.  
Space expansion or matter shrinks ?  
The search of a scaling model of the universe, a self-similar one or dilation, has been pursued by the scientific community since Dirac, Hoyle & Narlikar, and others without results.  
A scaling model is born, derived from data, using standard physics and making no hypotheses, this model has only one parameter ($H_0$) :  
A Self-Similar Model of the Universe Unveils the Nature of Dark Energy
So, from now on I'will ask for proper evidence (theoretical or experimental) that the atom is invariant every-time that I hear someone to say: the universe is expanding.  
Dark Energy? NO, imo it is an artifact of the reference we use (invariant atomic units).    
A: I may being hopelessly naive but if you substitute the claim "The farther out in space we look the faster the expansion" with the equivalent "The farther back in time we look the faster the expansion" then we might deduce Hubble's "constant" is reducing, which could be explained by gravitation, for example. It's just a question of overcoming the mental picture of the vast distance to high-Z galaxies by remembering that we are seeing a much smaller universe at that time. There wasn't room for "vast distances" then. which makes one wonder about inverse square law dimming for deducing distances as well as the effects on luminosity due to solid angle &c. 
