Dark Energy is really strange, but it accounts for about 72.1% of everything in the Universe, though in some cases theoreticians suggest it could be 75%!  A few years ago, every scientist would have agreed that though the Universe is expanding, the rate of the expansion was slowing.  When experiments were conducted to measure the rate of slowing, using a type of super nova in distant galaxies, the results astounded everyone.  The rate of expansion was increasing.  This is as though one were to throw a ball into the air, and instead of coming back down it just accelerated away! 

So; how did astronomers determine that the rate of expansion is increasing?  As I discussed in the section on Stars, a White Dwarf results from a low mass star that has stopped nuclear fission.  Depending on the size of the original star, a White Dwarf can have a range of masses, but there is
an upper limit beyond which they cannot remain stable.  This is called the Chandrasekhar limit.  Many White Dwarfs are in binary systems with a normal star, and draws matter from its companion star, which results in an increasing mass.  Once this reaches the Chandrasekhar limit, the star explodes in what is known as a Type 1a supernova.  This is very useful because, as this always occurs when the star has a known fixed mass, the luminosity of the explosion is always the same.  The apparent luminosity, when compared to the known absolute luminosity, tells us how far away the supernova occurred, and, therefore, how long ago it exploded.  Looking at the red-shift of the light tells us how much the Universe has expanded since the explosion.  When results from a great many type 1a supernovae were analyzed, it became apparent that the rate of expansion was increasing.  The reason postulated is Dark Energy that fills up space, and creates space between the galaxies forcing them apart.  If this continues, this would lead to an exponential expansion. 
The Cosmological Constant.  As this is a constant, it would mean that the rate of expansion must have been the same in the distant past.  The evidence, however, points to the rate of expansion increasing over time. 

Quintessence, is a scalar field, meaning that it is dynamic, and has a density and equation of state that varies over time.  There is an excellent discussion of quintessence at the Astronomy Today website. 

General Relativity, as postulated, fails over scales much larger than galactic superclusters.  General Relativity does not seem to be accurate at very small scales, for example at distance scales of the Planck length.  The question is, does it also fail to apply at very large distance scales? 
I guess we need to wait on more data.  I'll leave you with a quote from Edward Witten: “Dark energy has the somewhat unusual property that it was embarrassing before it was discovered”. 

Dark Energy

Astronomy & Cosmology -

Non-Baryonic Matter

Empty space contains energy as a result of quantum fluctuations. On the scale of the entire Universe, it could lead to a force that increases the rate of the expansion of the universe.  Or there could be some as yet unknown behavior at these small scales that we may find once we reconcile General Relativity and Quantum Physics. 

Dark energy could be a new force that only appears at very large scales, and that the Universe needed to grow before dark energy was able to overcome gravity.  It does appear that, as space expands, the affect of gravity between two bodies lessens, while the affect of dark energy stays the same or increases. 

As the Universe expands, the density of matter in it gets smaller as it becomes more spread out, while the density of dark energy is thought to remain constant.  Thus, in the past, the energy equivalent of the matter density was larger than the dark energy, so the rate of expansion of the Universe slowly declined.  As it expanded, reducing the density of matter, the amount of slowing of the rate of expansion reduced until, at some point, generally believed to have been about seven billion years ago, the energy equivalent of the matter density fell below the dark energy value.  As dark energy is a repulsive force, while gravity between matter objects is attractive, dark energy started to push matter apart more quickly as it overcame gravity.  Thus, the rate of expansion of the Universe has been slowly increasing over the last seven billion years. 
So; we do not know what comprises dark energy, but do we have any idea how it works?  There are several ideas; here are a few of them:

What comprises dark energy is unknown, but here are a few of the ideas circulating:
How Was It Found?
From What Is It Made?
How Does It Work?