It was originally thought that gravity would slow the expansion of the universe as huge astronomical bodies become attracted to each other and pull together like Sumo wrestlers trying to share a waterbed. That has not happened. Instead, the universe is expanding, and doing so at an increasing rate. Dark Energy is the general repulsive force that is kicking gravity's butt and driving the expansion of the universe.
The fact that it is called "Dark" Energy pretty much sums up how much we know about it. What's troubling about this lack of knowledge is that Dark Energy makes up about three-fourths of the known universe:
Figure courtesy of Wikimedia Commons
The main reason I find this troubling is because if Dark Energy is growing, then the Universe will keep expanding at a faster rate. Eventually everything in existence, including myself, will be pulled apart and destroyed by Dark Energy. Fortunately, researchers observing both optical and X-ray wavelengths have shown that Dark Energy is not increasing greatly over time, but more or less remains constant.
This is indeed important for the future of the universe. Even more important is the publicity focused on my cousin at Stanford, Brian Gerke, who was the first member of the family to get a PhD and a researcher in this area of astrophysics :
http://www.sciencenews.org/view/generic/id/39341/title/Dark_energy_constantly_with_us
There are somewhere between twenty and thirty Gerke cousins. Before long, we will have enough PhD's to take over the world...or at least Missouri.
Note: This piece was written in consultation with my sister Jill, a PhD student in astrophysics at The Ohio State University. She emphatically tells me that Dark Energy should not be definitively called either a force or a substance, because we just don't understand it. Hopefully I have worded this article carefully enough not to anger her or any other real physicists.
Hockey sticks do not lie, people.