The mystery of dark matter
/Wondering what space is made of? Well, so is everyone else.
It all started with Swiss astronomer Fritz Zwicky in 1933. He was measuring the speed at which the Coma galaxy cluster spun and noticed that the cluster was moving at a speed that implied much more mass than what was visible. Zwicky theorized there was some sort of “missing mass” keeping the galaxies from escaping the cluster’s gravitational pull.
In the 1970s, American astronomer Vera Rubin and her colleagues confirmed Zwicky’s theory. They observed that the mass of the stars within an average galaxy is only about 10 percent of what is required to keep those stars orbiting the galaxy’s centre.
Thus, the mystery of dark matter was born.
Composition
But what is all this “missing mass”? Space fog? Alien interference? Billions of tonnes of invisible primordial soup, the first ever forbidden snack? Maybe.
Image courtesy of unsplash
The truth is, we know more about what dark matter is not than what it is.
The first thing we know is that it isn’t visible to the eye. Dark matter is, well… dark. Its presence is detected by its gravitational pull rather than its luminosity or its ability to reflect light. This means that scientists need to get creative when it comes to measuring dark matter (but I’ll get to that later).
It also isn’t antimatter. Dark matter doesn’t produce the unique gamma rays antimatter does when it collides with regular matter.
Finally, it is not the same as the dark clouds of matter in and between galaxies. These gases are made up of baryons (protons, neutrons and the like), familiar and recognizable to any scientist with a degree in astrophysics. But a common belief is that dark matter isn’t baryonic at all. Instead, it’s assumed to be made up of particles that exist in theory but have yet to be observed.
That’s a longer way of saying we have no idea what this stuff is made of.
Measurement
It’s later, and now is the time to talk about measuring dark matter!
Scientists can indirectly measure dark matter through a method called gravitational lensing. Like with optical lenses, light passing through a gravitational lens gets bent, not unlike myself when I advise my brother to put anything but hot sauce on his vegetables.
When light from distant stars passes through a galaxy, the gravity of the matter in that galaxy causes the light to bend. As a result, the light looks like it’s coming from somewhere other than its actual origin. Many NASA scientists use the Hubble Space Telescope to observe the amount of bending to learn about the dark matter present in a certain galaxy or cluster.
image courtesy of unsplash
Theories
There are no small number of theories surrounding dark matter, but I’ll only cover some of the major ones.
Dark matter could be brown dwarfs—stars that never ignited because they lacked the necessary mass. Or maybe the remnants of the cores of dead small or medium sized stars, called white dwarfs. Dark matter might even be neutron stars, or giant galaxy-sized black holes, or literally magic.
Okay, probably not that last one.
The issue with those theories is that there just aren’t enough failed or dead stars to account for the amount of dark matter in the universe.
Dark matter makes up roughly 30 percent of the universe’s matter-energy composition (literally everything in the universe). I mean, that’s a substantial percentage, but it’s not that much, right? It may seem that way until you learn that everything on Earth and everything NASA has ever observed with all their fancy telescopes adds up to less than 5 percent of the universe. What a way to make you feel small.
The rest of the composition is dark energy, but that’s a whole other can of worms.
Conclusion
To this day, the nature of dark matter remains one of the greatest astronomical mysteries. Despite being aware of it for nearly a century, we know very few things about dark matter that aren’t (at least in part) theoretical. All we know for sure is that it’s there, and it’s keeping all those really pretty galaxies together. Everyone say thank you, dark matter!
Sources
https://www.britannica.com/science/dark-matter
https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy
https://www.nasa.gov/audience/forstudents/9-12/features/what-is-dark-matter.html
JAX YOUNG — Space enthusiast here to take you on a tour of the cosmos. Gets emotionally attached to Mars rovers. Virgo.
