Nov. 19, 2008: An international team of researchers has discovered a puzzling surplus of high-energy electrons bombarding Earth from space. The source of these cosmic rays is unknown, but it must be close to the solar system and it could be made of dark matter. Their results are being reported in the Nov. 20th issue of the journal Nature.

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"The source of these exotic electrons must be relatively close to the solar system—no more than a kiloparsec away," says co-author Jim Adams of the NASA Marshall Space Flight Center.

Why must the source be nearby? Adams explains: "High-energy electrons lose energy rapidly as they fly through the galaxy. They give up energy in two main ways: (1) when they collide with lower-energy photons, a process called inverse Compton scattering, and (2) when they radiate away some of their energy by spiraling through the galaxy's magnetic field." By the time an electron has traveled a whole kiloparsec, it isn't so 'high energy' any more.

High-energy electrons are therefore local. Some members of the research team believe the source could be less than a few hundred parsecs away. For comparison, the disk of the spiral Milky Way galaxy is about thirty thousand parsecs wide. (One parsec approximately equals three light years.)