Dark matter and energy are fascinating concepts that capture the imagination of every astrophysicist. They have not yet been observed directly, but they impact a lot of the processes we take for granted. Without them, galaxies would fly apart and possibly would not have formed. They account for 95% of the total mass-energy composition of the universe, with the rest being ordinary matter, so it is vital to talk about them. While some scientist proposed alternative explanations such as modifications to general relativity, the majority of the community agree that there is enough observational evidence to conclude the existence of dark matter and energy.
The first mentions of dark matter appear as early as 1884, when Lord Kelvin proposed that there must be dark bodies in the Milky Way, since the mass of the galaxy calculated using velocity dispersion of stars did not match up with the mass of visible stars. There were multiple other similar calculations that confirmed the disparity of the mass-light ratio. One of them was that the outside regions of galaxies were not supposed to be rotating as fast as they were if there was only ordinary matter. The name dark matter comes from the fact that this type of matter did not interact with light.
One of the major sources of evidence comes from gravitational lensing. This is when light rays coming from distant object get bent and distorted when passing over a gravitational well. Sometimes, rays get bet when passing over seemingly nothing, which indicates the presence of dark matter. This was observed in Abell 1689 and MACS J0416.1-2403 galaxy clusters. While a possible explanation would be the presence of brown dwarfs, neutron stars and supermassive black holes that are hard to detect, these would not account for enough of the missing matter.
What is it made of? It does not interact with baryonic matter other than through gravity. Therefore scientist hypothesise that it is a new elemental particle that has not been discovered yet. There are efforts at particle accelerators to detect it. A proposed particle is a sterile neutrino. The search for these weakly interacting massive particles is conducted on the Alpha Magnetic Spectrometer on the ISS and through the LUX detector.
In the 1920s and 30s, Edwin Hubble showed that the universe expands through the analysis of red shift in galaxies. Initially we thought that this rate is constant. Nonetheless the observations of distant supernovae in 1998 using the Hubble space telescope showed that the expansion was accelerating. This made no sense in the current model of physics, since we could reasonably expect that the rate would even slow down due to galaxies being gravitationally attracted to each other and eventually coming together in the center of mass of the universe. Therefore, there must have been a force acting against gravity and we have called it dark energy. While some scientist claim that this can be accounted for with Einstein’s cosmological constant from general relativity, others claim that it arises from a negative pressure field. We are yet to find out.
Dark matter and dark energy remain mysteries and are the main focus of cosmological research today. I believe we are closer to finding dark matter, nonetheless the search might require new technologies that we simply don’t have yet, or just a lot of luck. Studying these concepts is an exciting time in astrophysics, and hopefully we will have more than just computational answers soon.
References
Dark Matter and Dark Energy. (n.d.). Science. https://www.nationalgeographic.com/science/article/dark-matter
The Hubble Expansion. (n.d.). Berkeley Astronomy. https://w.astro.berkeley.edu/%7Emwhite/darkmatter/hubble.html
Magazine, S. (2010, April 1). Dark Energy: The Biggest Mystery in the Universe. Smithsonian Magazine. https://www.smithsonianmag.com/science-nature/dark-energy-the-biggest-mystery-in-the-universe-9482130/
Tillman, N. T. (2019, July 19). What Is Dark Matter? Space.Com. https://www.space.com/20930-dark-matter.html