These objects were first discovered by the Hubble Space Telescope in 1998. Before this time it was widely thought that the rate a which the universe was expanding was slowly decreasing from the rate at which it first started. However, as the Hubble Telescope observed supernova, it determined that the most distant had occurred 7.7 billion years prior. Using some complex math scientists determined that because this nova existed 7.7 billion years ago, the rate of expansion of the universe had to actually be accelerating. This puzzled scientists because there was no other force that could cause this that had been observed. So in order to explain this phenomenon they created Dark Matter and Dark Energy.
Dark Energy was what scientists used to explain the energy which was casing the universe to expand faster. Little is known about what causes this force, but there are some prevailing theories. One theory attributes Dark Energy as a property of space its self, which plays nice with Einstein's earlier gravity theories. However other theory's see Dark Energy as proof that our current understanding of gravity is completely wrong.
Dark Matter seems to be the more understood of the two mystery things. Dark Matter is mostly agreed to be the stuff that makes up the space in between real matter like stars, nebula, gas, and stuff like that. Although dark matter has just recently been discovered, scientists are confident in their understanding. This is because although dark matter is not directly visible, it can easily be found by the measurement of the way it bends and distorts light from distant galaxy's.
Its odd how much is unknown about dark matter and energy. Seeing that normal matter takes up only 4.6% of the universe you would think that more would be know about the more abundant dark matter (23.3%) and dark energy (72.1%). As more is discovered about these objects, the way we see our universe could dramatically change.