Question: What's dark matter?

Lucien Heurtier answered on 21 Mar 2023:

Dear past481hut ,

Clár-Bríd Tohill is very right!

Let me add a few things: When scientists started studying the velocity of stars in our galaxy, they noticed that a lot of star that are very far away from the center of the galaxy were going very VERY fast. They started wondering why, although they have so much energy, they are still turning around the galaxy. Take the Earth for instance: it is going quite fast around the sun, but exactly at the right velocity for it to orbit around it. If it would go slower then it would fall onto the sun. If it would go faster than it would fly away from the sun. It’s the same with stars around the galaxy. So with these very fast stars, scientists like Zwicky started to calculate if there is enough mass in the galaxy to keep these stars attracted to it (like there is enough mass in the Sun to keep the Earth orbiting around it). What Zwicky found is that there is not enough that we can see, by a LOT (that was in the 30’s). 40 years later Vera Rubin did very precise measurements and found out that in every galaxy she saw, there was the same problem: stars were turning very fast around them, but we don’t know what is keeping them attracted to their mother galaxy. That’s when people started believing in dark matter more.

During that time, people also realized that there was a strange microwave radiation that one could measure with telescopes in all directions of space, it is the so called Cosmic Microwave Background. It is basically some light that was emitted a long time ago and keeps arriving to us now, so it is like a very old picture of the Universe. In the beginning, telescopes were not good enough, so the picture they were obtaining using this light was pretty blurry. However, people believed that with a better resolution one should be able to see in this picture some big blobs corresponding to the energy that led to the formation of galaxies later on. So they developed greater telescopes to observe these blobs better. However, what they found is that … there were barely any blobs! Only very tiny ones! Then everybody wondered: how comes galaxies be formed if there was no blob of energy here and there??? Actually, if you put in a simulation a universe with barely any blob and only normal matter, you barely see any galaxy forming.

That’s where another piece of evidence for the existence of dark matter comes in: if you put the dark matter in these same simulations, then you see blobs forming much more efficiently. So you start with barely any blob, and very fast the dark matter helps to form some. And then you can obtain a universe like the one we observe today…

If you want to see that with your own eyes, I recommend that you look at this link:https://icc.dur.ac.uk/Eagle/highreader.php?page=evo

If you scroll down on this page, you can see a video with a box in it: it is the Universe. If you launch the video you see all the stars and galaxies forming. Above the video, you can click on stars, gas, or dark matter, and that will show you how each of these components evolves. Do it, it’s fun to watch the Universe evolve!

To finish, let me mention that there are many options for what dark matter could be. The most accepted answer is that it may be a particle: a particle that is very difficult to see because it does not reflect light, does not interact much with ordinary matter, except through gravitational forces. There are many theories that explain how it could have been formed. How it could be detected, and how heavy this particle is. One could fill a whole zoo with all the different theories that people came up with, but none of them has be proven right yet.

Dark matter could be made of other things than particles: It could also be made of small black holes, that are much lighter than the Sun, but still massive enough to have a sizeable effect on the formation of galaxies. There could be a lot around us, but we wouldn’t see them because they are black!

Finally, I should also mention that some people believe there is no dark matter, and would rather consider that the theory of gravitation is just wrong on very large distances. This is also possible, but difficult to accept as the theory of gravitation has predicted so many things with such a great accuracy, even in space… But we don’t have the answer yet. We need to wait for more detailed measurements from different telescopes and dark matter searches experiments. That’s why research requires sometimes a lot of patience… 🙂

I hope this helps!!

Cheers
Lucien