U.S. Department of Energy

Pacific Northwest National Laboratory


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Dark Energy

The surprising discovery in 1998 that the expansion of the universe is accelerating, instead of slowing down due to gravity, has posed another significant question: what is the dark energy that is pushing our universe apart? While it may be an inherent feature of the universe, it could be something dynamic related to new particles or forces, or a failure of Einstein’s theory of General Relativity. Dark energy may explain why the expansion of the universe continues to accelerate rather than slow down, as would be expected if gravity predominated. Both dark matter and dark energy remain among the top mysteries of the universe, key topics of astrophysical research today.

Dark Matter

In 1933, Fritz Zwicky, a faculty member of Caltech, coined the term “dark matter” to describe the unseen matter that must dominate the Coma Galaxy Cluster in order to match his observations of the motion of its galaxies. Scientists have been confident dark matter exists for more than 80 years, although they’re still trying to understand what it’s made of and why there seems to be so much of it. Dark matter is the gravitational glue that holds stars and gas in galaxies and great galaxy clusters together, whose fast-moving stars would otherwise be flung far apart. It is a form of matter that does not emit light and, therefore, is difficult to detect with ordinary observation methods. Scientists know it must exist, because its mass causes gravitational effects that can be observed. Based on the effects of gravity in our galaxy, scientists believe there is a high concentration of dark matter near the galaxy’s center around the supermassive black hole residing there.


Developed to identify, track and provide many details about subatomic particles, such as those produced by particle accelerators.