Editor's Note: We asked several scientists from various fields what they thought were the greatest mysteries today, and then we added a few that were on our minds, too. This article is one of 15 in LiveScience's "Greatest Mysteries" series running each weekday.
Scientists trying to create a detailed inventory of all the matter and energy in the cosmos run into a curious problem—the vast majority of it is missing.
"I call it the dark side of the universe," said Michael Turner, a cosmologist at the University of Chicago, referring to the great mysteries of dark matter and dark energy.
In fact, only 4 percent of the matter and energy in the universe has been found. The other 96 percent remains elusive, but scientists are looking in the farthest reaches of space and deepest depths of Earth to solve the two dark riddles.
Einstein's famous equation "E=mc^2" describes energy and matter (or mass) as one and the same—maps of the cosmos refer to the energy-matter combination as energy density, for short. The problem with detecting dark matter, thought to make up 22 percent of the universe's mass/energy pie, is that light doesn't interact with it.
But it does exhibit the tug of gravity.
Initial evidence for the mysterious matter was discovered 75 years ago when astrophysicists noticed an anomaly in a jumble of galaxies: The galactic cluster had hundreds of times more gravitational pull than it should have, far outweighing its visible mass of stars.
"We can predict the motions of the sun and planets very accurately, but when we measure distant things we see anomalies," said Scott Dodelson, an astrophysicist at Fermi National Accelerator Laboratory in Illinois. "Dark matter is currently the best possible solution, even though we've never seen any of it."
Another hallmark of dark matter is gravitational lensing, similar to the effect of light passing through a piece of polished glass. Massive objects like the sun can bend light, but colossal clouds of dark matter create "bubbles" in the cosmos that magnify, distort and duplicate the light of galaxies or stars behind them.
Gravitational lensing recently exposed evidence of the unseen mass in the Bullet cluster as well as in a ring around a cluster of colliding galaxies called ZwCl0024+1652.
In spite of the ghostly evidence, pieces of dark matter have yet to be pinned down by researchers. "Until we actually discover particles, we're not home yet," Dodelson said.
Particle physicists have detected neutrinos, which are extremely lightweight particles that pour out of the sun and hardly interact into ordinary matter, but Turner said they make up an extremely small fraction of dark matter in the universe.
"We arrested one of the members of the gang, but not the leader of the gang," Turner said of neutrinos. He thinks the leader is actually a WIMP: a weakly interactive, massive particle. Unfortunately, WIMPS are just a theory so far.
The thinking goes that WIMPs are very heavy, yet like neutrinos they rarely bump into matter to produce a detectable signal. But the idea that WIMPS—such as theoretical axion or neutralino particles—can bump into visible matter at all gives scientists hope.
"This is a story that may soon be at its end," Turner said, noting that the Cryogenic Dark Matter Search in the Soudan mine of Minnesota and other experiments below the ground should be sensitive enough to detect a WIMP.
Perhaps the biggest mystery of all is dark matter's big cousin, dark energy.
The invisible force is thought to be a large-scale "anti-gravity," pushing apart galactic clusters and causing the unexplainable, accelerating expansion of the universe. Turner thinks dark energy is the biggest mystery of them all—and quite literally, since physicists predict that it makes up 74 percent of energy density in the universe.
"So far, the greatest achievement with dark energy is giving it a name," Turner said of the elusive force. "We are really at the very beginning of this puzzle."
Turner described dark energy as "really weird stuff," best thought of as an elastic, repulsive gravity that can't be broken down into particles. "We know what it does, but we don't know what it is," Turner said.
While astrophysicists look deep into space to gather more details about dark energy's effects, Turner noted that theoretical physicists are focusing on explaining how the force actually works. And at this point, he joked, any physicist's explanation for dark energy is probably good enough to consider.
"We're at this very early stage, at the crime scene of dark energy's existence, if you will," Turner said. "It's a highly creative period, and now is the time for ideas."