Dark Energy is a (still) hypothetical form of energy that is supposed to permeate all space. It is almost like a sea of radiation, over which all normal matter (things that make us, stars and galaxies), normal energy (e.g. photons/light) and dark matter (termed such since it does not emit any light and thus cannot be detected) are like small islands. At large scales (larger than galaxies), according to General Relativity, dark energy acts as a force opposite to gravity, which causes the universe to expand at an ever-accelerating rate. Scientists had previously found evidence of dark energy billions of light years away, but a new computer model (coupled with observations from the Hubble Telescope) has found evidence of dark energy right in our own Milky Way galaxy :-).

Simulation: Red galaxies in a sea of dark energy (Courtesy: McMaster University)University of Washington professor Fabio Governato designed a computer model in 1997 that simulated the evolution of the universe from the big bang until the present. However, his model was unable to simulate the behavior of the galaxies in the local neighborhood of our Milky Way. By comparing his work to the recent observations by the Hubble telescope, he was able to add dark energy to his model, and voila! it was a perfect match!
This work suggests that there is dark energy in our Milky Way, and it is critically important in the life of a galaxy like ours. Slowly, but surely, the pieces of the cosmic puzzle are falling into place.