Last week, I talked about dark matter and how it is a placeholder name. Dark energy is essentially the same thing, only for a different problem.
When we look out at distant galaxies, we see something very surprising: they are all running away from us. At first glance, you may think that means that we are at the center of the universe. One thing you realize when you study science long enough is that this is a common first guess, but it has yet to ever be true. The reality is that all galaxies are running away from each other. This is because the universe is expanding.
Think of this like a balloon. Put just a little bit of air in the balloon and then draw dots all over the balloon with a marker. If you then continue to inflate the balloon the dots all move away from each other. From the perspective of any one dot it looks like all of the other dots are running away from it.
When scientists first discovered this expansion they expected that it would be slowing down due to gravity trying to pull everything together. What they found was that the expansion of the universe is speeding up. Dark energy is the place holdername for whatever it is that is speeding up this expansion.
Dark energy is literally one of the greatest mysteries in astronomy today. Some estimates say that it makes up 70% of our universe, yet we have no idea what it is. That is a big hole is our knowledge of the universe and I can't wait to see what fills it.
For more information on dark matter and dark energy you can check out NASA