Mysteries of the Universe

Hi, Science/Astronomy Enthusiasts,

Today, we are going to be learning about what Dark Matter is. Many people get Black Holes and Dark Matter confused, but they are totally different concepts in Astronomy, and in some aspects, very similar also. The structure of this post is going to be what my post in black holes was like, a little bit of background, and then its detailed definition with an explanation. Now get ready to get blown away from all the fascinating facts about Dark Matter!

Dark Matter's Background

You might be thinking, Dark Matter sounds like something dark, and while literary, it might not be "dark" after all. We still don't know what dark matter is, but we do know it exists and that it has some unique properties. Sounds strange right, wait for it, there are many more interesting things about this. In its simplest terms, Dark Matter is an unknown form of matter created to solve the Missing Mass Problem, but to further understand Dark Matter, we have to understand its background and its discovery, so let's come from the beginning.

In the 1960s and the 1970s, an astronomer named Vera Rubin, who studies and observes rotating spiral galaxies noticed something strange and off while calculating based on the principles taught by Johannes Kepler and Isaac Newton. In the 1600s, Kepler figured out that the further out a particular planet was from the sun, the slower it orbited around its sun. Newton actually put numbers into the calculations, calculating the strength of our sun's gravity, and by doing that, he could calculate the sun's mass. Rubin used the same principles, saying that by calculating the speed at which the gas clouds moved around the center of the serial galaxy, she could calculate the mass of the whole galaxy (She measured the gas cloud's velocities by using their Doppler shifts). Her prediction was that just like the planets further away from the sun would move slower than planets closer to the sun, the gas clouds further away from the center of the galaxy would move slower than the gas clouds closer to the center of the galaxy. But, her results said the opposite. She found that for many galaxies, "the farther out from the galaxy you went, the faster the clouds were moving!" [4] Even in other galaxies, the speeds should theoretically have declined, but instead flattened out with distance. This was pretty bizarre at the time because we could clearly see that there were more objects towards the center than in the edges but, the speeds did not decrease, increasing or staying consistent the further from the center of the galaxy. The only explanation was that there must be some "matter," perhaps "dark" and "invisible" matter we can't see contributing to the masses of the galaxies and, there must also be a lot of it. Rubin for example found that there must be 5 or 6 times as much of this "dark matter" as the visible matter in the galaxies. Even before Vera Rubin discovered dark matter in the 1960s, a Swiss astronomer named Fritz Zwicky also made a similar observation of when galaxies throughout were moving at the same speed, but his calculations were much of an overestimation, but the name he used got stuck to us till today, "Dark Matter."

What is Dark Matter

Now that we know a little bit of background, we can proceed to what dark matter actually us. But, trust me, this section is going to be short since right now, we have very very little, to virtually no idea on what Dark Matter is, but this is what we do know for now:

Dark Matter cannot reflect any light so pretty much, it would absorb any light that it received, or else, we would be able to detect it through telescoped or with just our naked eyes given that there would be a lot of it.
Dark Matter cannot give off any radio waves, or we would be able to detect it with radar or a fairly small and easy to obtain machine.
Dark Matter cannot give off radiation, or we would be able to detect it with an also fairly easy to obtain device like the Geiger Counter.
Dark Matter cannot be electrically neutral, or we would be able to detect it with a simple device also.
Dark Matter shouldn't give off anything that would fit into the Electromagnetic Spectrum, which includes everything from visible light and radio waves, to x-rays and gamma rays.
Dark Matter cannot even be made of Anti-Matter, or else the moment dark matter comes into contact with normal matter (which would probably be always), it would create a huge explosion that would release 100% of its stored energy (Since we don't randomly see explosions in the sky, we can rule this possibility out)

In fact, Scientists have determined that Dark Matter cant even be a normal matter, which means that whatever dark matter is, it cannot be made by atoms let alone protons, neutrons, or even electrons. Some scientists believe that dark matter might be this bizarre material called axions, which have a lot of properties similar to dark matter, but we don't know about that too.

If this thing called Dark Matter doesn't even interact with normal matter, how can we know for sure that Dark Matter actually exists just with the prediction and observations? We still need solid evidence, and while we still don't know for sure, there's another reassuring concept with dark matter that we can explore: Gravitational Lensing.

Gravitational lensing in action | ESA/Hubble

What is Gravitational Lensing?

In simple terms, gravitational lensing is basically a phenomenon that makes large, normal objects act like huge lenses. As mentioned in the Black Holes post, spacetime is a kind of fabric that everything with gravity bends, from galaxies to stars and even planets. Think of it as a real-life fabric, where a tennis ball is an object with mass, perhaps a planet. This bends the fabric of spacetime a little. Now, if marble is rolled to the side of the tennis towards it, it would bend while in the tennis ball's "gravitational field" and then return to its original path. Even concentrated masses can achieve the same effect. That's the same thing that would happen in space also, but the catch is that even light can bend too. So, what happens is that when an object or a group of objects emitting/reflecting light is so far away, the light dissipates, so we at earth cannot clearly see the object. But sometimes, these objects appear doubled to us on earth, and that's because of gravitational lensing, where something, either a perfectly placed planet or dark matter, bends the light from the objects and makes it come towards the earth.

Sometimes though, some objects appear to be gravitationally lensed without anything in the middle. Even though these predictions may be inaccurate, these might be the work of dark matter doing gravitational lensing.

That's it, Dark Matter at its simplest. We, humans, like to think that we have a somewhat special place in this universe, but the truth is we are just a tiny, tiny part of the whole universe. In fact, dark matter might be 85% of all the mass and objects of the universe, so in reality, we don't even know what most of the place we are even in! To learn more about Dark Matter, Gravitational Lensing, Spacetime, and much more, visit the websites linked below. Thanks a lot for taking the time to read this post, please visit again for other exciting posts just like this one, and until then, take care!

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