You may have heard about the excess of gamma-ray emission from the center of the Milky Way measured by the Fermi telescope. This excess can be interpreted as a signal of a 30-40 GeV dark matter particle - the so-called hooperon - annihilating into a pair of b-quarks. The inferred annihilation cross section is of order 1 picobarn, perfectly fitting the thermal dark matter paradigm. The story is not exactly new; the anomaly and its dark matter interpretation was first claimed 4 years ago. Since then there has been a steady trickle of papers by different groups arguing that the signal is robust and proposing dark matter or astrophysical explanations. Last week the story hit several news outlets, see for example here for a nice write up. What has changed that the anomaly was upgraded from a tantalizing hint to a compelling evidence of WIMP dark matter?
First, here is a bit more detailed description of the signal. The Fermi satellite measures gamma rays from all sky with a good angular and energy resolution. Many boring astrophysical processes produce gamma rays, for example cosmic rays scattering on the interstellar medium, or violent events happening around black holes and pulsars. However, known point sources, galactic and extragalactic diffuse emission, and the emission from the Fermi Bubbles do not seem to be enough to explain what's going on in the center of our galaxy. A better fit is obtained if one adds a new component with a spatial distribution sharply peaked around the galactic center and the energy spectrum with a broad peak near 2 GeV, see the plot. How much better fit? This paper quotes 40 sigma preference for this new component in the inner galaxy region. That's hell of a significance, even after translating the astrophysical sigmas to the ones used in conventional statistics ;)
Now, WIMP dark matter can easily reproduce the new component. Cold dark matter is expected to be sharply peaked near the galactic center, with the 1/r or similar profile. Furthermore, when dark matter annihilates into charged particles, the latter can radiate a part of their energy producing photons via the final state radiation, Compton scattering, and bremsstrahlung. This leads to emission of gamma rays with the energy spectrum depending on the dark matter mass and the identity of particles it annihilates into. Annihilation into leptons (electron, muons, taus) would produce a sharper peak than what is observed. As the plot shows, annihilation into quarks, whether the bottom or lighter one, fits the signal much better. All in all, the excess can be explained by a 15-40 GeV dark matter particle annihilating into quarks with the cross section in the 0.1-1 pb range.
This was known before, more or less. As far as I understand, the recent paper by Daylan et al. adds the following. They repeat the analysis using a subset of the Fermi data where the photon direction is more reliably reconstructed. This allows them to better study the morphology of the signal. They show that the excess is steeply falling (approximately as 1/r^1.4) all the way to about 2 kiloparsecs from the galactic center. Moreover, they demonstrate that the excess is to a good degree spherically symmetric. This can be regarded as an argument against conventional astrophysical explanations. For example, a school of several thousand milisecond pulsars could produce a similar energy spectrum as the excess, but would not be expected to be distributed this way.
Ah, and what does the Fermi collaboration have to say about it? As far as I know, there is no official statement about the excess. In this talk one finds the quote "[In the inner galaxy], diffuse emission and point sources account for most of the emission observed in the region". So we seem to have two slightly discrepant stories here: 40 sigma vs. nothing to see. If the truth were in the middle that would be great ;)
In any case, continuous emission from the galactic center will never be regarded as a convincing evidence of dark matter. To really get excited we would need to find a matching signal in a less messy environment. One possibility is the dwarf galaxies - small galaxies consisting mostly of dark matter that orbit the Milky Way. The Fermi collaboration recently reported the limits on the dark matter annihilation cross section based on observations of 25 dwarf galaxies, see the plot. Intriguingly, there is a small excess (global p-value 0.08) that may be consistent with the dark matter interpretation of the signal from the galactic centre... More data should clarify the situation, but for that we probably need to wait a few more years.