A century ago, astronomer Fritz Zwicky observed that galaxies were moving faster than their mass should allow, leading him to infer the presence of an invisible structure, dark matter. Since the particles that make up dark matter do not interact with electromagnetic force, they cannot be observed directly, as they do not absorb, reflect, or emit light.
Now, NASA’s Fermi space telescope has found specific gamma rays in the center of the Milky Way that are consistent with the decay of theoretical dark matter particles, although they could also come from other sources.
“If this is correct, to my knowledge, it would be the first time that humanity has ‘seen’ dark matter,” said study author Tomonori Totani in a press release. The article is published in the Journal of Cosmology and Astroparticle Physics.
Miguel Ángel Sánchez Conde
Professor and researcher in the Department of Theoretical Physics at the Autonomous University of Madrid and at the Institute of Theoretical Physics (IFT UAM-CSIC), scientific coordinator of the entire NASA Fermi-LAT Collaboration between 2023-2025 and coordinator of the ‘Dark Matter and New Physics’ working groups of the Fermi-LAT gamma-ray collaborations (2014-16 and 2020-2022) and CTAO (2018-2020) collaborations
Autonomous University of Madrid
Institute of Theoretical Physics UAM-CSIC
Spanish National Research Council (CSIC)
Via Science Media Centre Spain
This work is part of the community’s efforts to unravel the nature of dark matter—undoubtedly one of the greatest enigmas in science today—based on indirect signals in astrophysical data. In this case, the study focuses on WIMPs, by far the most studied candidate to date and the community’s favourite. WIMPs are expected to annihilate in particularly dense areas of the universe, such as our own galaxy, giving rise to Standard Model particles that we can search for, such as gamma rays, the most energetic form of light in the universe and the subject of this particular work. After a couple of decades of uninterrupted searching with our gamma-ray telescopes, we still do not have a clear signal from these WIMPs, although there has been and continues to be some potential evidence over the years, which has not been fully confirmed despite our efforts.
Professor Totani’s research uses gamma-ray data collected by NASA’s Fermi satellite in the direction of intermediate regions of our galaxy, which are analysed and interpreted in a robust manner using completely standard tools in the field. The work is therefore of good quality and contains abundant checks that can be reproduced and interpreted in the context of searches for dark matter in the form of WIMPs. The study contains some interesting new developments compared to previous work (for example: the use of more years of data, an alternative treatment of some components of the diffuse galactic emission in the adjustments, a prior spatial smoothing of the data). If confirmed, the finding would have enormous repercussions both within and outside the community. It would undoubtedly be one of the great discoveries in the history of science.
Unfortunately, despite being a serious and highly noteworthy piece of work, its conclusions are currently subject to considerable uncertainty, making it impossible to claim that ‘this is the first time dark matter has been observed’. These uncertainties are the result of our still very limited knowledge about the exact production of gamma rays through conventional astrophysical phenomena in different areas of our galaxy. The study claims that these “conventional” processes are insufficient to explain the excess gamma radiation observed and that, therefore, this excess is due to the annihilation of WIMPs. However, there are very significant degenerations between the different components that we know contribute to diffuse galactic emission in this energy domain. This has led in the past to similar claims on several occasions that we had detected dark matter, when in all those cases a more detailed/complete study concluded that it was incorrectly modelled “conventional” astrophysics. The exception is the excess gamma-ray emission observed in the galactic centre, the origin of which is still unknown to us some fifteen years after its discovery (it is still a real possibility that it is due to dark matter). It is therefore not 100% certain that i) the aforementioned excess signal exists, given the current uncertainties in the modelling of diffuse emission, and that ii) this excess, if it exists, is due to dark matter and not to some other astrophysical component not yet considered.
Other equally relevant issues cast additional doubt on the interpretation of the observed excess in terms of dark matter. To explain this excess with WIMPs, as proposed in the paper, they would have to annihilate at a rate approximately ten times higher than expected (that is, if we wanted to explain all dark matter in the form of WIMPs). This high annihilation rate also seriously conflicts with the most robust estimates we currently have for WIMPs, which we have derived from observations of dwarf galaxies (the best objects for this type of search). The excess understood as WIMP annihilation would also require the distribution of dark matter in the galaxy to be particularly atypical and unexpected, as it would imply a sudden change in the areas closest to the galactic centre, so as not to conflict with our gamma observations in that area.