In late 1998, a group of scientists claimed to have proof that they witnessed an apparent acceleration of the expansion of the universe is attributed to a new form of energy. This was discovered during a routine (if you can call such events routine) 10-year study of supernovae.

The Lawrence Berkeley National Laboratory's Department of Energy had an international Supernova Cosmology Project. The focus was to find as many of the supernovae as possible, as they were deemed to be too random in their occurrence to be found via some systematic search. Early attempts at using Chaos Theorem to predict supernovae appearances turned out to be non effective.

By 1998, the Supernova Cosmology Project (part of the Berkeley effort) and the Mount Stromlo Research School of Astronomy and Astrophysics had recorded several dozen supernovae, including some so distant that their light had started toward Earth when the universe was only a fraction of its present age.

Coming up with a methodology and a definitive measurement of changes in the expansion rate of the universe was their primary mission. It was envisioned that this would facilitate a better understanding to the origin, state, and fate of the cosmos.

The surprise

Human beings think in terms of terrestrial physics, which often do not apply within a quantum universe lexicon. Most of us would assume that if the big bang theorem was in fact true, that the universes expansion would likely gradually slow and reverse. This misperception is largely due to our limited understanding of gravitational forces affecting properties of terrestrial kinetic energy (think: What goes up, must come down). The original plan of the researchers from the SCP and Mount Stromlo projects was to calculate the deceleration of the universe and come up with an estimation of its age, future and definitive consensus on the average density of matter in the universe.

The largest surprise everyone got was to discover that the universe's expansion was not decelerating at all, it was accelerating. The force that facilitates this behavior has been blessed with the moniker "Dark Energy".

The new form of energy was deemed "Dark Energy" and appears to reside in space. One of it's primary properties is that it can balance the effects of kinetic energy arising from the expansion of the universe. The end result is a zero spatial curvature in space. Several of the possible paths for the universes ultimate demise or growth are depicted in the graph below (courtesy of the Supernova Cosmology Project)

Efforts to detect this phenomena have resulted in an observation mapping fluctuations in the cosmic microwave radiation background. If the dark energy is a constant (the so-called
cosmological constant) or growing then the fate of the universe is sealed: it will continue
expanding forever. If the level (or the net effect) of the dark energy is waning,
then it was even more important in the past, and may have played a part in limiting the
formation of the largest gravitationally bound structures.

My personal take on this all

I wonder if anyone has considered that the apparent acceleration of the expansion of the universe at it's outer edges is an effect of captured energy dating back to T-zero. Perhaps, as in terrestrial explosions, some of the matter managed to capture more direct energy from the birth of the universe. These particles of matter may display apparent acceleration when compared to other particles. If the linear speed, unchecked by gravity, continues to increase, it could be attributed to matter reacting to stored energy within the core of the larger chunks of matter, much the same way a golf ball's liquid core causes it to accelerate after it has been hit. With no opposing forces, perhaps it is racting in ways that terrestrial matter cannot.

Someone please feel free to shoot holes in this theorem - after all, I am merely an amateur physicist.

Links to read more:

http://www.lbl.gov/supernova
http://www.lbl.gov/Science-Articles/Research-Review/Highlights/1998/PHYS_cosmos.html
http://universe.gsfc.nasa.gov/science/darkenergy.html