Dark energy itself is a controversial topic. It is a hypothetical form of energy that acts opposite to gravity over very large distances. It is used by some physicists to explain the accelerating expansion of the universe and the “missing matter” in the universe.
From the article: “Outside the ‘surface’ of a dark-energy star, it behaves much like a black hole, producing a strong gravitational tug. But inside, the ‘negative’ gravity of dark energy may cause matter to bounce back out again.”
The paper was originally published (PDF) in the Texas Conference on Relativisitc Astrophysics, 12/12-12/17/04.