The astronmer Fritz Zwicky first proposed the idea of dark matter in 1933 when he observed that the relative motion of galaxies within glalactic clusters could not be explained solely by the presence of luminous matter, i.e. stars. There had to be something else contributing to the gravitational forces. Since that time more and more evidence has piled up in favor of this idea. According to modern astrophysical observations the amount of the stuff exeeds that of luminous matter by a factor of ten! There are various theories about the composition of dark matter, for example, a kind of elementary particle that interacts very weakly with ordinary matter. Sensitive earth-based experiments are being carried out to try to detect the individual constituents of dark matter but up to now no convincing evidence has been found.
The idea of dark energy is a relatively new, although Einstein had a term in his famous general relativiy equations that could mimic the effect. The recent observational evidence comes partly from looking at the excessive dimming of supernovae stars as a function of their distance, or red-shift, from the earth. The effect can be explained as an increase in the Hubble expansion rate as a function of time. This can, possibly, be explained by ascribing to space itself a property called dark energy. It's pretty weird. The idea is fairly new and many astronomical observations must be made in order to confirm it.
Check out the Wikipedia articles on dark matter and dark energy for more information.
There are also many excellent Scientific American articles in the last few years on these topics. Several other lines of evidence, in addition to the red-shifts, support the idea that some sort of dark energy is driving an accelerating expansion. The pattern of random wiggles in the intensity of cosmic microwave radiation from different directions provides one such line of supporting evidence.
(published on 10/22/2007)