Partner Series

No matter how astrophysicists crunch the numbers, the universe simply doesn't add up. Even though gravity is pulling inward on space-time — the "fabric" of the cosmos — it keeps expanding outward faster and faster. To account for this, astrophysicists have proposed an invisible agent that counteracts gravity by pushing space-time apart. They call it dark energy.

Scientists think that dark energy permeates all space and comprises some 70 percent of the universe. However, despite its supposed prevalence, it has never actually been detected or observed, and so its nature remains unknown. Scientists just assume that dark energy must exist. But why do they think so?

The universe is expanding. What's more, it seems to be expanding faster all the time. This was discovered independently in the late 1990s by Saul Perlmutter and other members of the Supernova Cosmology Project at Lawrence Berkeley National Laboratory, and Adam Riess and Brian Schmidt of the High-z Supernova Search Team at Harvard; both teams were both looking at light coming from distant, exploding stars called supernovas . (On Oct. 4, Perlmutter, Riess and Schmidt were jointly awarded the 2011 Nobel Prize in Physics for their work.)

Loosely speaking, as light travels toward us through expanding space, it gets stretched. This causes it to become "redshifted," or redder in color. Light coming from supernova explosions far away is redshifted more than light from closer ones, because the former has traveled a greater distance through the expanding universe than the latter, and for a longer time.

What the physicists observed, though, is that light from the closest supernovas is disproportionately redshifted; it has undergone more than its fair share of stretching. Because this light left its source more recently, this implies that the universe is now expanding faster than it was long ago. In other words, the expansion of the universe must be accelerating. All the stars, galaxies and galaxy clusters are moving faster and faster apart.

But why? If gravity alone was acting on the matter in the universe, we wouldn't expect expansion to speed up, because gravity pulls massive objects toward each other. Apparently there is another effect in place. Something exists that is driving space (and the matter within it) apart.

This is the something that physicists call dark energy. Based on the observed rate of expansion, scientists know that the sum of all the dark energy must make up about 70 percent of the universe.

Since 1998, the supernova data has been corroborated by other evidence — most notably by measurements of the cosmic microwave background by the WMAP satellite, which showed that about 70 percent of the contents of the universe are some undetected form of energy.

A huge number of theories exist on the nature of this elusive substance. In the most widely accepted of them, dark energy is a "cosmological constant" — an inherent property of space itself, which has "negative pressure" driving space apart. As space expands, more space is created, and with it, more dark energy.

Real or a mirage?

Since the 1998 discovery, other theories have been put forth that provide alternative explanations — including ones that don't require dark energy to exist — for the group's observations of light from supernovas and the cosmic microwave background. For example, a theory proposed by Christos Tsagas, a cosmologist at Aristotle University of Thessaloniki in Greece, suggests that the movement of our local bubble of the universe relative to the rest of it could induce an acceleration in the expansion of our bubble. This would make it seem like the universe recently began accelerating (because light from nearby would be disproportionately red-shifted compared to light from far away) when in fact the universe as a whole is actually decelerating. [The Accelerating Universe and Dark Energy Might Be Illusions ].

Perhaps the truth or falsity of dark energy and the accelerating universe will be proven in the coming months. The Dark Energy Camera, run by the Dark Energy Survey, an international collaboration of research institutes and universities, will soon turn on at the Cerro Tololo Inter-American Observatory in Chile and begin searching for dark energy in distant galaxies.

Incidentally, if you want to learn more about dark energy, feel free consult Wikipedia's entry on the subject. For an article on the accuracy of Wikipedia , Riess told us that the dark energy page was "remarkably accurate."

Follow Natalie Wolchover on Twitter @nattyover. Follow Life's Little Mysteries on Twitter @llmysteries, then join us on Facebook.