Dark Matter in the Centre of our Home Galaxy 

Fermi telescope made clean observation of excess γ-ray emission at the centre of our home galaxy which appeared non-spherical and flattened. Referred as Galactic Centre Excess (GCE), this excess γ-ray is a possible signature of dark matter arising as a product of self-annihilations of weakly interacting massive particles (WIMPs), a dark matter particle candidate. However, the excess γ-ray observed at the galactic centre may also be due to old millisecond pulsars (MSPs). Hitherto, it was held that the GCE morphology due to dark matter (DM) would be spherical. A recent simulation study reveals that gamma-ray morphology due to DM could be significantly nonspherical and flattened. This means both dark matter (DM) annihilations and millisecond pulsars (MSPs) hypotheses for the observed GCE are equally possible. The gamma rays produced in the annihilation of dark matter (DM) would have an extremely high energy level of approximately 0.1 tera-electron-volts (TeV). The standard gamma-ray telescopes cannot detect these high-energy photons directly. Hence, confirmation of dark matter (DM) model of Galactic Centre Excess (GCE) would be possible upon completion of studies by the tera γ-ray observatories like Cherenkov Telescope Array Observatory (CTAO) and Southern Wide-field Gamma-ray Observatory (SWGO).

The story of dark matter began in 1933 when Fritz Zwicky observed that the fast-moving galaxies in the Coma Cluster cannot hold together and remain stable without presence of additional matter that is somehow invisible but exerted adequate gravitational effect to stop galaxies from falling apart. He coined the term “dark matter” to refer to such invisible matter. In the 1960s, Vera Rubin made a seminal contribution towards our understanding of dark matter. She noted that the stars at the outer edges of Andromeda and other galaxies were revolving with a speed as fast as the speeds of the stars towards the centre. For the given sum of all observed matter, the galaxy should have flown apart necessitating presence of some additional invisible matter that keep the galaxies together and cause them to rotate at high speeds. Her measurements of rotation curves of Andromeda galaxy provided earliest evidence of dark matter.  

Now we know that dark matter does not interact with light or electromagnetic force. It does not absorb, reflect or emit light or any other electromagnetic radiations and is invisible hence referred as dark. But it clusters gravitationally and have gravitational effect on ordinary matter, and this is how its presence in space is generally inferred. Galaxies are held together in equilibrium by the gravitational effect of the dark matter which constitutes as much as 26.8% of the mass energy content of the universe whereas the entire observable universe including all the baryonic ordinary matter that we all are made up of make only 4.9% of the universe. The remaining 68.3% of the mass energy content of the universe is dark energy.  

It is not known what dark matter really is. No fundamental particles in the Standard Model have properties needed to be dark matter. Perhaps, hypothetical “supersymmetric particles” that are partners to the particles in the Standard Model make dark matter. Perhaps there is a parallel world of dark matter. WIMPs (Weakly Interacting Massive Particles), axions, or sterile neutrinos are hypothesized particles beyond the Standard Model that are leading candidates. However, no success has been achieved yet in the detection of such particles.  

There are several projects (such as XENON Experiment, DarkSide-20k Project, EURECA Rxperiment, and RES-NOVA) currently underway for direct detection of dark matter particles. These are mostly liquid noble gas detectors or cryogenic detectors which are designed to detect faint signals from interactions of dark matter particles. However, despite many novel approaches, no project has been able to directly detect any dark matter particle yet. 

For the indirect evidence of the dark matter, one may look for gravitational effects of dark matter, as Fritz Zwicky and Vera Rubin did to discover dark matter by studying how galaxies are held together despite having speeds disproportionately high for the observed ordinary matter. The gravitational effects of lensing (bending of light) and effects on movement of stars in space can also provide indirect evidence of presence of dark matter. In addition, annihilation products (such as gamma-rays, neutrinos, and cosmic rays) created when dark matter particles collide with each other in space can also indicate presence of dark matter. One such location where dark matter was predicted based on products of annihilation of dark matter particles is the centre of our home galaxy Milky Way.  

Detection of dark matter in the centre of our home galaxy Milky Way  

There were indications of an excess diffuse microwave central glow at the centre of Milky Way (MW). The excess glow was proposed to be due to synchrotron emission from relativistic electrons and positrons generated in WIMP dark matter annihilation, hence an extended diffuse γ-ray signal in the energy range up to a few hundred GeV was predicted. Subsequently, the Fermi-Large Area Telescope (LAT) detected the γ-ray signal which was identified as the Galactic Centre Excess (GCE). Soon, it was realised that the Galactic Centre Excess (GCE) could also be due to old neutron stars (millisecond pulsars). It was thought that the morphology of the GCE would be important – a symmetrical spherical-shaped GCE would be indicative of γ-ray emission from annihilation of dark matter (DM) particles while a flattened morphology of GCE would be suggestive of γ-ray emission from millisecond pulsars (MSP).  

Extensive observation of the Milky Way galactic centre by Fermi-Large Area Telescope (LAT) revealed a flattened asphericity. Ordinarily, one would associate the observed asphericity to old stars (MSP) however a recent study published on 16 October 2025 has concluded that the GCE morphologies predicted by both old stars (MSP) and dark matter (DM) annihilation models are indistinguishable.   

To study distribution of the dark matter, the researchers conducted simulation of the morphology of MW (Milky Way)-like galaxies. They found that the dark matter halos around the galaxies as well as around the central regions of the galaxies were rarely spherical as assumed in anisotropic model. Instead, the analysis showed a flattened dark matter density projection for all the galaxies. This non-axisymmetric dark matter (DM) distribution was shown also by the merging history of Milky Way galaxy in the first three billion years in the history of universe. The observed morphology of GCE is flattened over the central region, which is generally thought to be characteristic of old star (MSP) distribution. The new study has demonstrated that the dark matter (DM) generates a similar boxy distribution. Thus, both dark matter (DM) annihilations and millisecond pulsars (MSPs) hypotheses for the observed GCE are equally possible.   

Whether the observed GCE is due to dark matter (DM) or due to millisecond pulsars (MSPs) would be known when γ-ray observatories like Cherenkov Telescope Array Observatory (CTAO) and Southern Wide-field Gamma-ray Observatory (SWGO) complete their tera-gamma ray studies in future. The gamma rays produced as an annihilation product of dark matter (DM) in the galactic centre would be ultra-high-energy photons with an extremely high energy level of approximately 0.1 tera-electron-volts (TeV). Standard gamma-ray telescopes cannot detect these high-energy photons directly. Tera-gamma rays are going to be an important target for future γ-ray observatories like CTAO and SWGO.  

This study is a step forward in the detection of dark matter in space through its annihilation products however presence of dark matter at the galactic centre would require confirmation by the ultra-high energy γ-ray observatories such as CTAO or SWGO in future. Much more significant progress in the science of dark matter would be direct detection of any DM particle.  

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References:  

1. Hochberg, Y., Kahn, Y.F., Leane, R.K. et al. New approaches to dark matter detection. Nat Rev Phys 4, 637–641 (2022). https://doi.org/10.1038/s42254-022-00509-4 

2. Misiaszeka M. and Rossib N. 2024. Direct detection of dark matter: a critical review. Symmetry 2024, 16(2), 201; DOI: https://doi.org/10.3390/sym16020201  

3. Instituto de Física Corpuscular. In search of dark matter: a new approach to detecting the invisible. 22 August 2025. Available at https://webific.ific.uv.es/web/en/content/search-dark-matter-new-approach-detecting-invisible 

4. Muru M.M., et al 2025. Fermi-LAT Galactic Center Excess Morphology of Dark Matter in Simulations of the Milky Way Galaxy.  Physical Review Letters. 135, 161005. Published 16 October 2025. DOI: https://doi.org/10.1103/g9qz-h8wd . Preprint version at arXiv. Submitted 8 August 2025. DOI: https://doi.org/10.48550/arXiv.2508.06314  

5. Johns Hopkins University. News – Mysterious glow in milky way could be evidence of dark matter. Posted on 16 October 2025. Available at https://hub.jhu.edu/2025/10/16/mysterious-glow-in-milky-way-dark-matter/  

6. Leibniz Institute for Astrophysics. News – Milky Way shows gamma ray excess due to dark matter annihilation. Posted 17 October 2025. Available at https://www.aip.de/en/news/milkyway-gammaray-darkmatter-annihilation/  

7. Fermi Gamma-ray Space Telescope. Available at https://science.nasa.gov/mission/fermi/  

8. Cherenkov Telescope Array Observatory (CTAO). Available at https://www.ctao.org/emission-to-discovery/science/  

9. The Southern Wide-field Gamma-ray Observatory (SWGO). Available at https://www.swgo.org/SWGOWiki/doku.php?id=swgo_rel_pub  

10. Tartu Observatory. Dark side of the Universe. Available at https://kosmos.ut.ee/en/dark-side-of-the-universe 

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