Dark matter is a hard-to-detect substance that is believed to make up a large part of the known universe. It is called "dark" because it does not emit or reflect any kind of radiation and does not interact with normal matter. Therefore, it cannot be observed directly. The leading scientific theory is that dark matter may consist of weakly interacting massive particles or WIMPS. However, no WIMPS have been directly observed to date.Cosmic Variance: Dark Matter Exists

Dark Matter was first observed in 1933 by CalTech astronomer Fritz Zwicky.Swemorph: Fritz Zwicky Subsequent observations and measurements have provided good indirect evidence for the existence of dark matter, and the scientific evidence continues to grow. However, there is not yet a scientific consensus on the substance.Science Daily: New Detector Will Aid Dark Matter Search (December 8, 2008)

Another type of unknown substance in the universe is called dark energy. It is believed to make up 74% of the known universe. Like dark matter, it cannot directly be detected. However, scientists believe that dark energy exists because of the way the universe works. For example, without dark energy, the universe would have stopped expanding and begun contracting, but instead, the universe continues to expand outward at an accelerated rate. Furthermore, the growth of galaxies and clusters is slower than predicted, which scientists attribute to dark energy.TG Daily: New dark energy findings suggest Einstein's theory is dead-on (December 16, 2008)

On December 16, 2008, scientists released research that strongly suggests dark energy actually exists. Using the Chandra X-ray Observatory, they made detailed observations and calculations to support the dark energy theory. Although other theories still exist, some scientists believe that these observations add more evidence to prove Einstein's Theory of Relativity.TG Daily: New dark energy findings suggest Einstein's theory is dead-on (December 16, 2008)

• YouTube & Google Video: Dark Matter

• Google Video: Theories of Everything: Dark Matter

• NASA: Image of the Day, Dark Matter

• Google Image Search: Dark Matter

• Yahoo Image Search: Dark Matter

• YouTube: Dark Matter Bob McDonald Interview

• YouTube: NOVA scienceNOW | The Dark Matter Mystery | PBS (Time 4:37)

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