Dark energy is a mysterious form of energy pervading the Universe that is causing its expansion to accelerate. Of all things in astronomy, we probably know the least about it, but this is in part due to it only being discovered in 1998! So there hasn't been time to figure it all out.
But that doesn't mean we know nothing about it, and the folks at Hubble have put out a really good documentary about dark energy on their site. I watched the whole thing through, and it does a good job describing how it was discovered, what we know about it, what we don't know about it, and where this investigation will lead. It features interviews with astronomers Saul Perlmutter and Adam Riess, who were instrumental in discovering this weird energy, and as an added bonus some fun quotations from astronomers who didn't believe the results when they were first released.
I can see this documentary being useful to the public as well as in classrooms. If you're a teacher than you should really take a look at it.