What is the Relationship Between Dark Matter and Dark Energy?
Dark matter and dark energy comprise about 96% of the total matter-energy in the known universe. While dark matter and dark energy are not the same, they do share a few features. Neither principle can be either observed or measured by any of the tools or techniques that are currently at the scientific community's disposal. Instead, both the concepts of dark matter and dark energy serve as place holders for basic principles of the universe whose features and functions remain unknown. One principle, dark matter, helps hold things in place through the force of gravity. The other principle, dark energy, is a force of unknown origin that is causing space to expand at an increasing rate.
Leonard Susskind lecture http://www.youtube.com/watch?v=3SiGujnfDVc
Patricia Burchat TED talk http://www.ted.com/talks/patricia_burchat_leads_a_search_for_dark_energy.html