Shedding Some Light on Dark Matter

Author: Zaid Amer

Have you ever been in a Gravitron? They’re a type of ride, common at fairs across the world, that spin incredibly fast to exert pressure on you to stick to the side of their sloped walls. These walls keep you from spinning out of the machine and injuring yourself a couple feet away. Just like the Wilde Knight Mares ride in Canada’s Wonderland!

Well, believe it or not the same thing is happening to galaxies across the universe. They’re all spinning really, really fast, with the average galaxy rotating at 240 km/s, and the fastest at 560 km/s!¹ That’s like travelling across the Great Wall of China and back in 20 seconds! This sounds (and is) incredible, but there's just one problem. The galaxies are spinning much, much faster than they should be. They’re spinning so fast they should tear themselves apart.

Gravity helps keep things together—it's the force across the universe that attracts any two objects with mass to each other. However, the observable mass (mass that scientists can measure) within these galaxies is less than it should be for galaxies to stay attracted around their center. ² At the speeds the galaxies rotate, they should have pieces ripping off, getting flung out into space. But that clearly doesn’t happen, and that’s why we came up with a phenomenon called dark matter. At least, it’s why we “think” dark matter exists.

We say “think” because despite more and more measurements about the cosmos coming out in support of a theory like dark matter, we’re unsure of what’s really happening because dark matter is invisible. ³ That’s the reason it’s called “dark” matter. But by not interacting with light or producing any energy, we currently have no way of directly measuring dark matter.

We only know it exists from supporting evidence that shows there should be a lot more matter than we can see (like the speed galaxies spin at). Another piece of evidence comes from an increase in gravitational lensing. ⁴ Gravitational lensing is a phenomenon in which an object with a large enough mass will warp spacetime [Definition: a mathematical model that joins space and time into a single idea called a continuum, named that because there are no missing points in space or instants in time]. Warping it to such a degree that all light is bent when travelling through that area. But what does that mean?

Let’s imagine spacetime as a rubber sheet, stretched on a ring like a trampoline, with balls moving across it. Depending on the weight, or the mass of a ball, spacetime will be impacted to different degrees. The bigger the ball, the bigger the impact on space time. Now, when we add a new ball to roll across this sheet, it will be impacted by the “gravitational” effects of the other objects. What’s interesting is that astronomers have realized, after careful scrutiny, that the mass required for the level of gravitational lensing that we measure, is much more than the amount of mass that we can actually see. There must be something to account for this difference in mass that’s “hidden.”

Thus, we know dark matter exists (in theory at least). Most astronomers agree on its high probability by its observable effects on the cosmic web, but it has yet to be confirmed by any of the scientific tools we have available for measurement. Which is even more shocking when considering physicists best estimate dark matter accounts for roughly 85% of the mass of the universe. ⁵ That’s a lot of mass to be hiding out in space!

The quest to try and identify these dark matter particles has not been in vain though, since they have allowed for many scientific innovations, from improving hypersensitive sensors to advancing algorithms and quantum computing. ⁶ But, despite all the work and millions of dollars spent, we have yet to find strong enough evidence of dark matter, except for agreeing that the cosmos shouldn’t exist as it does, unless something like dark matter exists or something even more mysterious…

1. NASA. “Super Spirals Spin Super Fast.” HubbleSite, October 19, 2019. https://hubblesite.org/contents/news-releases/2019/news-2019-54.

2. Tillman, Nola Taylor. “What Is Dark Matter?” Space.com, January 28, 2022. https://www.space.com/20930-dark-matter.html.

3. Ibarra, Alejandro. “Dark Matter Theory.” Nuclear and Particle Physics Proceedings 267–269 (October 2015): 323–31. https://doi.org/10.1016/j.nuclphysbps.2015.10.126.

4. Sutter, Paul. “How Much of the Universe Is Dark Matter?” Space.com, December 6, 2022. https://www.space.com/how-much-of-universe-is-dark-matter.

5. Sutter, “Universe and Dark Matter”.

6. Department of Energy. Doe Explains...Dark Matter, n.d. https://www.energy.gov/science/doe-explainsdark-matter.