Back in the 20th century, it was generally accepted that in the begining, or at least the most recent begining, there was a singularity. Then space started to expand, rapidly, which was known as inflation, and the universe was born. Back in these early moments the universe was essentially a plasma, but as the universe expanded, the matter cooled and then later started to coalesce, resulting in stars and galaxies. When all this matter started to group together, gravity started to weigh in and the expansion of the Universe began to slow.
At least, that's what everyone believed. Unfortunately, that last bit was not true. In fact, it turns out that the expansion of the universe is accelerating, not slowing down. Having no clue what was going on, scientists have labeled the phenomenon "Dark Energy" as in "There must be some Energy that we can't see that's overcoming the gravitational attraction, we'll call it Dark Energy".
Now the scientists that brought this discovery to light have been awarded the $1Million Shaw Astronomy Prize. The Shaw Prize committee said the discovery "...has profound implications that may revolutionize our understanding of the nature of the physical world and its ultimate fate. It is an achievement that is already taught to schoolchildren."
http://www.photonics.com/content/news/2006/June/23 /83160.aspx