Galaxies are a plethora of light and baryonic matter that are intertwined through the Tully-Fisher relation, but the composition of these spiral galaxies goes beyond what can be seen. This unseen introduces us space enthusiasts to a world of non-baryonic matter and halos, something that claims 70% of our universe’s mass. We are talking about the aesthetically sketched, Dark Matter. 

The majority of visible matter present in a spiral galaxy is clustered at its core, giving it a dense centre. Newtonian laws of movement suggest that as we move farther away from the center of galaxies, the orbital velocities of the stars that embellish these galaxies should slow down, following a Keplerian decline. Vera Rubin (the mother of Dark Matter) brought forward a stupefying result in her galaxy rotation rates. She found that galaxy rotation curves fell flat even as the radii from the center increased (Scoles, 2023). Stars in galaxies were orbiting at nearly constant speeds, even at the edges, where there was little visible matter. This discrepancy could only be explained by an unseen gravitational reservoir. This elusive ghost was labelled as Dark Matter. 

This ocean of gravity and mass is inferred not only in galaxy rotation curves but also in gravitational lenses. According to Einstein's law of relativity, mass bends and warps the fabric of spacetime, causing light to deviate and change its direction. When light from a distant galaxy or quasar passes through a massive object– such as a galaxy cluster– the light is distorted and magnified, creating beautiful arcs and rings called gravitational lenses as described by the European Space Agency. 

By studying these lensing effects, astronomers can create a 3D map of mass distribution in space. Time and again they find much more mass than what is visible, suggesting the presence of Dark Matter halos (Cooper, 2025). According to the Max Planck Institute for Astrophysics’ article, “The Millennium Simulation”, computer simulations of nuclear biosynthesis like the Millennium Simulation Project reveal that clusters and galaxies could not have coalesced in such a short period of time without the influence of a cosmic glue that gravitationally holds matter together. Cold Dark Matter in this case acts a lot like gluons agglutinating quarks together. Cold Dark Matter isn't literally cold, it is non-relativistic and does not interact with photons or baryonic matter (Scott, 2013, p. 22). Its insentient nature towards electromagnetic radiation tells us that Dark Matter is not made of protons, electrons, and neutrons. 

Scientists have been speculating on probable candidates for Dark Matter building blocks, here are the most popular options:

  1. Weakly interacting massive particles (WIMP): These are hypothetical thermal relics of our universe that only interact through gravity. The idea behind WIMP Dark Matter is that these particles, if they exist, should occasionally collide and annihilate, producing a cascade of familiar particles—including antiprotons. If this were happening in significant amounts, we’d expect to see a noticeable excess of cosmic ray antiprotons (Kamionowski, 1997).

    1. However, the PAMELA experiment (Adriani et al., 2009) found no such excess. Instead, the number of antiprotons detected closely matched what we’d expect from known astrophysical processes, like high-energy cosmic rays colliding with interstellar gas.

  2. Axioms: These are light bosons originally proposed to resolve quantum chromodynamics (QCD) problems, manifesting as ultra-light Dark Matter. In this scenario, halos resemble quantum fluids, their density oscillating on scales influenced by wave interference (Kamionowski, 1997).

The Cuspy Halo Problem presents another hitch within our understanding of Dark Matter. Dark Matter simulations predict that these extended non luminous masses take the shape of nonhomogeneous cusps with maximum density at the centre (illustrated by the Navarro Frenk White Profile). But N-body simulations portray a flatter central Dark Matter density profile in dwarf galaxies (Gilmore & Read, 2005). Numerous solutions have emerged to this discrepancy, one of them being cusp-flattening baryonic feedback.

Pontzen & Governato (2012) propose that repeated bursts of star formation and subsequent supernova explosions create rapid fluctuations in the gravitational potential. These fluctuations transfer energy to Dark Matter particles, pushing them outward and flattening the density profile. Another solution is the presence of what physicists like to describe as warm-fuzzy-meta or self-interacting Dark Matter. 

Dark Matter is not just a question of missing mass, it is a question of how we do science. The essence of scientific inquiry is not to stagnate ourselves by believing what seems certain, but to challenge, refine, and sometimes overturn our best theories. The presence of Dark Matter is one of the most debated topics in cosmology, not because we lack evidence of its gravitational effects, but because science demands that every assumption be tested, every conclusion be scrutinized. 

Some see the missing mass problem as proof of an undiscovered particle, others as a sign that our understanding of gravity itself is incomplete. Each new observation brings fresh data, yet no final answer—only better questions. That is where science thrives.

References

  1. Adriani, O., Barbarino, G., Bazilevskaya, G. et al. (2009). An anomalous positron abundance in cosmic rays with energies 1.5–100 GeV. Nature, 458, 607–609, https://doi.org/10.1038/nature07942 

  2. Bertin, G. (2022). Visible and dark matter in the universe: A short primer on Astrophysical Dynamics. Cambridge University Press. https://doi.org/10.1017/9781009023368 

  3. Cooper, K. (2025). Scientists find hints of the dark universe in 3D maps of the cosmos. Space.com. Retrieved February 21, 2025, from https://www.space.com/the-universe/galaxies/scientists-find-hints-of-the-dark-universe-in-3d-maps-of-the-cosmos

  4. European Space Agency. (n.d.). Gravitational lensing. European Space Agency. Retrieved February 21, 2025, from https://esawebb.org/wordbank/gravitational-lensing/#:~:text=Gravitational%20lensing%20occurs%20when%20a,accordingly%20called%20a%20gravitational%20lens

  5. Gilmore, G., Read, J. I. (2005). Mass loss from dwarf spheroidal galaxies: the origins of shallow dark matter cores and exponential surface brightness profiles, Monthly Notices of the Royal Astronomical Society, 356(1), 107–124, https://doi.org/10.48550/arXiv.astro-ph/0409565 

  6. Governato, F., & Pontzen, A. (2012). How supernova feedback turns dark matter cusps into cores, Monthly Notices of the Royal Astronomical Society, 421(4), 3464–3471, https://doi.org/10.1111/j.1365-2966.2012.20571.x 

  7. Kamionkowski, M. (1997). WIMP and axion dark matter. 1997 ICTP Summer School on High Energy Physics and Cosmology, Trieste, Italy, June 2--July 4, 199. https://doi.org/10.48550/arXiv.hep-ph/9710467 

  8. Moffat, J. W., & Toth, V. T. (2013). The dark matter problem: A historical perspective. Physics Reports, 605, 1-99. https://doi.org/10.1016/j.physrep.2013.07.003 

  9. Navarro, J. F., Frenk C. S., & White, S. D. M. (1996). The Structure of Cold Dark Matter Halos, International Astronomical Union, 171, https://doi.org/10.48550/arXiv.astro-ph/9508025 

  10. Scoles, S., (2023). How Vera Rubin confirmed dark matter https://www.astronomy.com/science/how-vera-rubin-confirmed-dark-matter/ 

  11. Scott, P. (2011). Searches for Particle Dark matter: An introduction. arXiv.org. https://arxiv.org/abs/1110.2757 

  12. Virgo Consortium. (n.d.). The Millennium Simulation. Max Planck Institute for Astrophysics. Retrieved February 21, 2025, from https://wwwmpa.mpa-garching.mpg.de/galform/virgo/millennium/

  13. Weakly interacting massive particles. (n.d.). https://www.scientificlib.com/en/Physics/LX/WeaklyInteractingMassiveParticles.html 

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