Neil deGrasse Tyson offers one of the simplest, most straightforward summaries of one of the greatest challenges and questions facing modern Science today.
In astronomy and cosmology, dark matter is a hypothetical form of matter that is undetectable by its emitted electromagnetic radiation, but whose presence can be inferred from gravitational effects on visible matter. According to present observations of structures larger than galaxies, as well as Big Bang cosmology, dark matter and dark energy could account for the vast majority of the mass in the observable universe.
In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe. Dark energy is the most popular way to explain recent observations and experiments that the universe appears to be expanding at an accelerating rate. In the standard model of cosmology, dark energy currently accounts for 74% of the total mass-energy of the universe.
Dr. Neil deGrasse Tyson is an American astrophysicist and the Frederick P. Rose Director of the Hayden Planetarium at the American Museum of Natural History in New York City. An accomplished scientist, he is also outspoken for his rationalist and skeptical world-views, and was honored with the 2009 Isaac Asimov award from the American Humanist Association.