What Are Dark Matter and Dark Energy?

WMAP launched in 2001, but the dark energy problem presented itself before -- back in 1998 when the Hubble Space Telescope observed three very curious supernovae. The most distant of these cosmic blasts occurred 7.7 billion years ago, more than halfway back to the big bang itself [source: Hubblesite].

This insight into the ancient cosmos revealed that the universe's expansion hasn't been slowing, but rather accelerating. This rapid expansion threw astronomers for a loop, most of whom had assumed before this revelation that the expansion had slowed over time due to gravity. Scientists attribute this accelerating expansion to dark energy, so called because its exact nature remains a mystery. However, something must fill the vast reaches of space to account for the accelerating expansion.

We might not know what it is just yet, but dark energy exists -- and scientists have a few leading theories. Some experts believe it to be a property of space itself, which agrees with Einstein's cosmological constant theory on gravity. This theory argues that dark energy is so integral to space that it wouldn't dilute as the universe expands.

Einstein's cosmological constant theory is invested in another curious term: vacuum energy. Composed of empty space, vacuum energy is believed to be a key driver of accelerated expansion in the universe. Since vacuum energy density is so similar in nature, it's often interchangeably called dark energy.

Another theory, which has been partially disproven, defines dark energy as a new type of matter. Dubbed "quintessence," this substance would fill the universe like a fluid and exhibit negative gravitational mass [source: NASA]. Other theories involve the possibilities that dark energy doesn't occur uniformly, or that our current theory of gravity is incorrect.

Dark matter, by comparison, is far better understood. It doesn't emit or reflect light, but scientists can estimate where it is based on its gravitational effects on surrounding matter. Scientists use a technique called gravitational lensing to accomplish this, observing the way dark matter's gravitational pull bends and distorts light from distant galaxies.

These observations rule out stars, antimatter, dark clouds or any form of normal matter. Some scientists consider supermassive black holes a potential candidate for dark matter, while others favor either MACHOs (massive compact halo objects) and WIMPs (weakly interacting massive particles). MACHOs include brown dwarfs, weak stars that exert a gravitational pull but emit no light. WIMPs, on the other hand, would constitute a radically different form of matter left over from the big bang.

Research into the exact nature of dark matter and dark energy continues. In what appears to be empty space, dark energy dominates, and it's vital to understand how this matter dominated universe functions. In due course, scientists hope to discern a clearer understanding of these two glaring (or dark), cosmic unknowns.