Such an inventory is far, far removed from the time, not very long ago, when humans considered themselves not only alone in the cosmos, but also the center of creation. It turns out, based on current knowledge, the cosmos has no center. It more resembles the surface of an expanding balloon, on which every point is equivalent to every other.

The humbling process has not ended. In recent years, scientists have observed that visible matter represents only a tiny fraction of the mass of the universe - less than 5 percent. The rest of existence seems to be made of two components: dark matter and dark energy, which comprise, respectively, about 25 percent and 70 percent of the universe.

Next to nothing is known about dark energy, except it seems to be strong enough to be accelerating the expansion of the universe, pushing it toward an utterly cold and silent end.

Dark matter is beginning to be another story. Still theoretical - scientists so far have not been able to observe direct evidence of its existence - the possibility of dark matter first emerged because of a puzzling phenomenon.

When astronomers began measuring both the rotation rates and probable masses of galaxies, they were surprised to discover the two figures did not match. The outer stars of galaxies seemed to be orbiting at speeds that should have sent them careening into space. Yet they remained captured by gravitational forces that appeared far too weak to contain them.

Dust and black holes were not the answer, either, because they, too, were not massive enough to generate enough gravity. Something else had to be supplying the missing mass. Hence, the theory of - and search for - dark matter began.

New research by scientists at the University of Zurich, Switzerland, and published in the Jan. 27 issue of the British journal Nature, constitutes the latest attempt at discovery. Their computer models suggest halos of dark matter - weighing as much as Earth but as large as the whole solar system - emerged as the first structures of the nascent universe.

Such halos were pervasive. The Milky Way galaxy alone may contain quadrillions of them, the Swiss scientists wrote. One passes by Earth every few thousand years or so. In doing so, it leaves a bright - and detectable - trail of gamma rays in its wake.

Even without the halos, they added, countless random dark matter particles rain down upon the Earth, passing through our bodies undetected.

"These dark matter halos were the gravitational glue that attracted ordinary matter, eventually enabling stars and galaxies to form," said theoretical physicist Ben Moore, co-author of the Nature paper.

Moore and colleagues in Zurich based their months of supercomputer calculations on a newly named particle called a neutralino.

"Until 20 million years after the Big bang, the universe was nearly smooth and homogenous", he said. Then, slight imbalances in the distribution of matter allowed gravity to work.

Previous theory held that the imperfections jolted the universe's equilibrium, creating little pockets with greater and lesser densities of matter. The more dense pockets attracted more and more matter, until enough accumulated to create the first stars. The process continued through the universe's 13.7-billion-year history, yielding the galaxies, galaxy clusters and galaxy super-clusters visible today.

The Zurich work imposes an intermediate step. The supercomputer model showed how neutralinos created in the Big Bang first formed gravitational wells in space, into which ordinary matter flowed. After about 500 million years, the first galaxies appeared.

Neutralinos apparently were ideal candidates for galactic formation. They do not move very fast and clump together easily, enough to create the gravitational wells - the seed pods of future galaxies.

The calculations by the Zurich group have uncovered several intriguing possibilities:

- The Earth-mass halos formed first, in gigantic numbers.

- The structures seem to be composed of extremely dense cores, a property that has enabled billions upon billions of them to have s urvived for the entire age of the Milky Way.

- Miniature versions of the halos continue to move through the galaxy and interact with ordinary matter via gravity as they pass by.

It might even be possible, the Nature authors said, that the halos could disturb comets residing in the Oort cloud, far beyond Pluto's orbit and send objects plunging through the solar system toward the sun.

"Detection of these neutralino halos is difficult, but possible", said Juerg Diemand, another co-author of the paper and one of the supercomputer's designers and builders. That is because the halos constantly emit gamma rays, the highest-energy form of light, he said. The computer model predicts the neutralinos generate gamma rays when they collide and self-annihilate.

"A passing halo in our lifetime would be close enough for us to easily see a bright trail of gamma rays," said Diemand, who is now at the University of Calif ornia, Santa Cruz.

A quicker way to detect neutralinos, he said, would be to look at the galactic center, where dark-matter density is highest. The denser regions provide a greater chance of neutralino collisions and therefore more gamma-ray emissions.

Still, Diemand said, "this would still be difficult to detect, like trying to see the light of a single candle placed on Pluto."

In 2007, NASA plans to launch its Gamma-ray Large Area Space Telescope, or GLAST, which will search for the emissions if they exist. New ground-based gamma-ray observatories, such as VERITAS or MAGIC, also might be able to detect gamma rays from neutralino interactions in dark-matter halos.

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