This weekend the featured plot is borrowed from the presentation of Brandon Anderson at the symposium of the Fermi collaboration last week:
It shows the limits on the cross section of dark matter annihilation into b-quark pairs derived from gamma-ray observations of satellite galaxies of the Milky Way. These so-called dwarf galaxies are the most dark matter dominated objects known, which makes them a convenient place to search for dark matter. For example, WIMP dark matter annihilating into charged standard model particles would lead to an extended gamma-ray emission that could be spotted by the Fermi space telescope. Such emission coming from dwarf galaxies would be a smoking-gun signature of dark matter annihilation, given the relatively low level of dirty astrophysical backgrounds there (unlike in the center of our galaxy). Fermi has been looking for such signals, and a year ago they already published limits on the cross-section of dark matter annihilation into different final states. At the time, they also found a ~2 sigma excess that was intriguing, especially in conjunction with the observed gamma-ray excess from the center of our galaxy. Now Fermi is coming back with an updated analysis using more data and better calibration. The excess is largely gone and, for the bb final state, the new limits (blue) are 5 times stronger than the previous ones (black). For the theoretically favored WIMP annihilation cross section (horizontal dashed line), dark matter particle annihilating into b-quarks is excluded if its mass is below ~100 GeV. The new limits are in tension with the dark matter interpretation of the galactic center excess (various colorful rings, depending who you like). Of course, astrophysics is not an exact science, and by exploring numerous uncertainties one can soften the tension. What is more certain is that a smoking-gun signature of dark matter annihilation in dwarf galaxies is unlikely to be delivered in the foreseeable future.