Music Music 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. Humm and beep Hum and beeping Hum and beeping