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For the past 40 years, astronomers have known that something about the
cosmos doesn't add up. First in galaxy clusters
and then within individual galaxies, they found that visible matter
--stars, gas and dust--cannot account for motions they observe.
No one knows what this missing mass, now
called "dark matter," actually is, but studies by NASA's
WMPA spacecraft of the cosmic microwave background--the oldest
light in the universe--show how much is out there. Dark
matter outnumbers ordinary matter by 4 to 1.
The WMAP results also hint that dark matter likely takes the form on an
as-yet-undiscovered subatomic particle. WIMPs
represent one hypothesized class of these particles. They
neither absorb nor emit light, and don't interact strongly with other particles.
But when they encounter each other, they annihilate and make gamma rays.
That's where NASA's Fermi Gamma-ray Space 
Telescope comes in. Two years of scanning the sky with Fermi's
Large Area Telescope have set the strongest limits yet for WIMP dark 
matter. The best place to look for gamma rays from dark matter annihilation?
The most boring galaxies around, called dwarf spheroidals.
These faint, tiny galaxies possess impressive 
amounts of dark matter, but they contain no gamma-ray-emitting objects,
and little gas or star formation. In the currently accepted
cosmology, the first structures formed as the gravitation of dark matter
corralled normal matter. Simulations show that the largest structures
formed in this way were comparable to the dwarf spheroidal galaxies we see 
today. It's thought that large galaxies like our own were
built-up from collisions among these dwarfs.
Using two years of data, Fermi scientists explored ten dwarf 
galaxies for an sign of gamma rays from WIMP annihilation. In
this graph, the dashed line marks the sweet spot where conventional expectations
for WIMP dark matter align with what we know about our universe.
Even when scientists combine all of the Fermi data from all ten of the
dwarfs, they see no sign of gamma rays. This limit shrinks
the box where WIMP-based dark matter may be found, and for the 
first time, shows that the cosmology we know essentially eliminates some
WIMP types. The longer Fermi operates, the better
its ability either to box in the nature of dark matter, or to find actual
evidence of what it is. And the discovery new dwarf galaxies will
make this search even more sensitive. Although 
nondescript, dwarf spheroidal galaxies may have been the first large
structures to form in the universe. Now, they've taken
center stage in the drama to solve one of astronomy's greatest mysteries.
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