Dark matter makes up approximately 85% of the total matter in the universe, but we do not know what it is [1]. The other 15% is ordinary matter, such as protons, electrons and so on. This document will explore the nature of dark matter filaments and the cosmic web. Also, it will look at the methods that can be used to investigate these phenomena.

Figure 1: A picture of the Cosmic Web from The Millennium Simulation Project. The figure clearly shows the filamentary structure that is etched out in the early universe. Source: [1]
The difference between ordinary and dark matter is that ordinary matter does interact with the electromagnetic force (the same force that attracts opposing magnets or repels two protons) and dark matter does not [2]. Therefore, this means that dark matter does not interact with light, as light is a consequence of electromagnetic interactions, and hence why it is called ‘dark matter.’ This poses challenges in detecting dark matter as it cannot be directly observed. We have only implied its existence through measuring its gravitational effects on galaxies [3]. What we do know is that the dark matter in the early universe formed filamentary (thread-like) structures and this allowed the formation of a web throughout the cosmos (the cosmic web) [2]

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Just after the big bang, the temperature and density of ordinary matter was high due to the universe being very compact [2]. The particles that constitute ordinary matter collided very frequently via the electromagnetic force [2]. Thus, areas of higher density would be pushed apart by this force, preventing regions from growing in mass [2]. This is similar to how highly pressurized gas will disperse when in a region of lower pressure.
At the same time, dark matter, which does not interact with the electromagnetic force [5], could have areas of higher density. These regions could grow by pulling in more dark matter using the gravitational force [2]. Ultimately, this allowed the dark matter to form large structures that varied in density throughout space. Consequently, this created a vast network of thread-like structures that consisted of dark matter [2].
Once ordinary matter had cooled as the universe expanded, particles, such as protons and electrons, could form neutral atoms allowing areas of increased density to occur [2]. This matter began to fall into the existing dark matter structure as outlined above. Additionally, ordinary matter can get hot and emit radiation, allowing it to fall deeper into the areas of high density [2]. As a result, this created a highly filamentary structure of ordinary and dark matter that is known as the cosmic web [2]. At the intersections of this structure, where the matter is most dense, stars and galaxies formed to create the universe we know today [2]. Figure 1 shows a simulation of the cosmic web that is etched out in the early universe [1].
Simulations of the evolution of the universe inform us of the processes involved in the formation of the cosmic web. Once the simulations have been completed, they can be compared against observational data to determine their accuracy. The Millennium Simulation Project is an example of a simulation that can tell us the physical processes involved in the formation of the universe and how the cosmic web was formed [6]. Specifically, it has been fundamental in predicting the conditions required for the cold dark matter model to be true. This is just one example of how we can investigate the cosmic web and determine some properties of dark matter.
Another example of how the cosmic web and dark matter filaments can be researched is measuring gravitational lensing caused by these filaments [7]. Gravitational lensing is the effect that occurs when light bends due to the force of gravity exerted onto it [7]. This phenomenon is described by general relativity, but this is beyond the scope of this review. It can be used to measure the mass of a celestial object [7]. Additionally, as lensing occurs due to the force of gravity, it can be used to imply the existence of dark matter filaments and the cosmic web [8]. This is because the filaments are massive enough to affect the light passing near it to a measurable degree. Thus, gravitational lensing can be used to map the cosmic web and show us the distribution of dark matter throughout the universe [7].
In summary, the properties of an unknown matter, known as dark matter, have shaped the universe into a web-like structure called the cosmic web [2]. This structure contains a sub-structure of many massive filaments and at the intersection of these filaments galaxies and stars have formed [2]. The scientific community has endeavoured to map this structure to learn more about the universe. This can be done using the concept of gravitational lensing that allow us to indirectly imply the existence of the cosmic web [3]. Looking to the future, more research, such as the Millennium Simulation Project, is required to continue developing the theories and models that could explain our universe, specifically the nature of dark matter and the cosmic web.
References
[1] V. Springel, “The largest N-body simulation of the universe,” 2 8 2004. [Online]. Available: https://wwwmpa.mpa-garching.mpg.de/mpa/research/current_research/hl2004-8/hl2004-8-en.html. [Accessed 17 10 2019].
[2] “History of Cosmic Structure Formation,” [Online]. Available: https://www.esa.int/Our_Activities/Space_Science/Planck/History_of_cosmic_structure_formation. [Accessed 6 October 2019].
[3] Z. Merali, “Dark matter’s tendrils revealed,” 4 July 2012. [Online]. Available: https://www.nature.com/news/dark-matter-s-tendrils-revealed-1.10951. [Accessed 6 October 2019].
[4] “Electromagnetic force,” Plasma-Universe.com, [Online]. Available: https://www.plasma-universe.com/electromagnetic-force/. [Accessed 3 November 2019].
[5] “Dark Matter,” CERN, [Online]. Available: https://home.cern/science/physics/dark-matter. [Accessed 03 November 2019].
[6] S. D. M. W. A. J. C. S. F. N. Y. L. G. J. N. R. T. D. C. J. H. J. A. P. S. C. P. T. H. C. A. E. J. C. &. F. P. Volker Springel, “Simulations of the formation, evolution and clustering of galaxies and quasars,” Nature, vol. 435, p. 629, 2005.
[7] E. Grocutt, “What is Gravitational Lensing?,” [Online]. Available: http://www.cfhtlens.org/public/what-gravitational-lensing. [Accessed 6 October 2019].
[8] I. Czekala, “Strands in the Cosmic Web,” 8 July 2012. [Online]. Available: https://astrobites.org/2012/07/08/strands-in-the-cosmic-web/.
[9] E. Lerner, “Cosmologists weigh cosmic filaments and voids,” 17 April 2014. [Online]. Available: https://phys.org/news/2014-04-cosmologists-cosmic-filaments-voids.html. [Accessed 6 October 2019].