Dark Matter and Dark Energy
There are two kinds of unknown matter-energy in the universe. One of these is called dark matter and the other is called dark energy. Dark matter might just be relatively ordinary matter that is just not emitting light. So, burned-out stars might be contributing to dark matter. Or it could be that dark matter is a form of matter associated with previously unknown particles which have a large mass but do not otherwise interact very strongly. These hypothetical particles are called WIMPS for weakly interacting massive particles. WIMPS have yet to be seen, although experimental searches are underway.
Dark Matter is thought to contribute to about 23% of the mass of the known universe. This dark matter appears to attract ordinary matter and as such would be acting to decrease the expansion of the universe. Dark matter would thus be gravitic rather than antigravitic. Dark matter may possibly be totally unconnected with dark energy, but this is not exactly known, only hypothesized about.
Dark matter also appears necessary to account for the fact that galaxies do not fly apart even though the energy associated with their angular motion exceeds their deduced gravitational (binding) energy. In essence, an attractive and unseen (dark matter) appears necessary in order for things to work out when it comes to galactic mechanics. More on Dark Matter Here
Note: The hypothetical constituent elements of dark matter are also sometimes referred to as MACHOS ... More On MACHOS here
Dark energy is energy that appears to fill the universe and appears to have an antigravitic nature in that it is thought to be pushing the universe apart. It is thus reasoned that the rapid rate of expansion of the universe is a consequence of dark energy. But no one as of yet knows what dark energy is. Some people think that dark energy is a manifestation of Einstein's cosmological constant. This would be associated with a very large negative energy density that might be seen everywhere in space. Einstein was the first to introduce the cosmological constant when he was developing his general theory of relativity and later said that this was his greatest mistake [ref]. 73% of the mass-energy of the universe is said to be related to dark energy. [ref]
-- R. Koontz
Series of Articles On Dark Matter and Dark Energy Link
Dark Energy and Dark Matter Video
Study Sheds Light On Dark Energy Link
Dark Energy and the Runaway Universe Link
Patricia Burchat: The Search for Dark Energy and Dark Matter Link