The Astronomist On The Dark Sector.

The Astronomist has a pretty good write up on dark energy or "the dark sector", highlighting the main ideas. (I encourage everyone to read it.)  He even has a derivation of the 10^120 order of magnitude problem and some hypothesis for what is going on.

Personally, I believe dark energy is just a cosmological constant.  I think this is what best fits the data.  We've measured w several times to be ~-1and more and the error bars just keep shrinking around that value. (For those who don't know w, or the ratio between pressure and density of the substance,  equaling -1 is the prdiction for dark energy being a cosmological constant.)

That said, we aren't 100% sure.  Moreover, as The Astronomist points out, if it is a cosmological constant we still need to understand why it is so small.  This is a major unresolved issue for physics.