Ted.com will inform on why they think there is dark matter & dark energy

dark matter can be proven by looking at a cluster of close galaxies (ones that aggregate because of this dark matter). Around the perifery is a smear of parts of a circle of light. This is light from behind the cluster being deflected by an unseen mass, because the mass is spherical (ish) and light that radiates , radiates in all directions and that which would just miss us gets bent back to us from everywhere in 2 dimensions. a speakers at TED.com explains it better.

dark energy is explained by the way the universe is expanding, the best theories can't explain it, so they have pencilled-in dark energy until Einstein Mk2 comes along.

Patricia Burchat: Shedding light on dark matter - she explains it more accurately than I.