WASHINGTON - In ``Star Wars,'' Darth Vader rules the ``dark side'' of a fantasy universe. In real life, astronomers are exploring the ``dark side'' of our own universe. They find it a mystifying place.

According to a batch of new reports published in a special ``Welcome to the Dark Side'' issue of the journal Science, most of the cosmos cannot be seen, even with the most powerful telescopes. All but a tiny fraction of creation consists of two exotic, invisible ingredients called ``dark energy'' and ``dark matter.''

Astronomers admit they don't understand either of them.

``Cosmologists have no idea what the nature of the dark matter and the dark energy may be,'' Jordi Miralda-Escude, an astronomer at Ohio State University in Columbus, wrote in Science.

``We're stuck with this preposterous universe,'' said John Carlstrom, an astrophysicist at the University of Chicago. ``It's a universe in which ordinary matter, the stuff of which humans, stars and galaxies are made, accounts for less than 5 percent of the universe's total mass and energy.''

According to the latest astronomical observations, normal atoms make up only 4 percent of the stuff in the universe. Dark matter, which usually is assumed to be invisible subatomic particles left over from the birth of the universe, accounts for 23 percent. The remaining 73 percent consists of dark energy -- a weird, repulsive force sometimes called antigravity or negative gravity -- which is making the universe expand faster than it used to.

These proportions have changed over the 14 billion years since the big bang, the reports in Science say. Until about 7 billion years ago, dark matter was dominant, and the tug of its gravity gradually was slowing down the expansion of the universe. Eventually, however, dark energy took command, and has been speeding up the expansion ever since.

In other words, dark matter acts as a brake on cosmic expansion, dark energy as an accelerator.

For decades, theorists speculated that much of the universe was beyond their ken. Only in the past few years have observations on extremely sensitive telescopes confirmed that speculation and clarified a few of its details. Various experiments are under way, or proposed, to unravel the many remaining mysteries.

Although dark matter cannot be seen, its presence can be detected by the effect of its gravity on nearby stars and galaxies, Jeremiah Ostriker, an astrophysicist at Princeton University, reported in Science.

Recently, Aaron Lewis, an astronomer at the University of California-Irvine, reported that observations by an X-ray telescope revealed a gigantic halo of dark matter, weighing as much as 100 trillion of our suns, surrounding a cluster of galaxies known as Abell 2029, a billion light-years from Earth. (A light-year is about 6 trillion miles.)

Last month, Spanish astronomer Francisco Prada announced that only massive concentrations of dark matter could explain the motions of 3,000 galaxies his team had surveyed.

Astronomers think dark matter was responsible for the creation of the first galaxies some 13 billion years ago: Its gravity pulled together ordinary atoms floating loosely in the cosmos until they formed an object massive enough to form stars and shine.

``Dark matter is now known to be the vital ingredient in the cosmos, six times more abundant than ordinary matter,'' Ostriker wrote. ``Without dark matter, the universe would have remained too uniform to form the galaxies, stars and planets. We would not exist today were it not for dark matter.''

Dark energy -- or negative gravity -- is even more baffling than dark matter. Its existence wasn't established until 1998, when observations of exploding stars called supernovae revealed that the expansion of the universe is speeding up.

Additional supernovae studies have ``boosted these results from a startling possibility to conventional wisdom in just the past five years,'' said Robert Kirshner, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

``We should be humble about dark energy,'' said Sean Carroll, a University of Chicago astronomer. ``We haven't a clue as to what is going on.''

Some physicists theorize that a vacuum, like empty space, is actually a seething mass of unknown particles that continuously pop in and out of existence, creating pressure that drives the expansion of the universe.

Anthony Tyson, an astrophysicist at Bell Laboratories in Murray Hill, N.J., told Science: ``The universe is not those pinpoints of light we can see in the night. It is, in fact, this dark side.''

For more information on the Web, go to: http://chandra.harvard.edu/

xray_astro/dark_matter.html or www.space.com/scienceastronomy/dark_matter_021112.html.