Welcome back!

I hope you are all safe and doing well.

We are coming up on our first year complete for this blog! It’s been a tremendous journey and I wouldn’t have been able to do it without your help and support.

In today’s blog post, I thought I’d return to the outer reaches of space, touching on a topic that many people find confusing—understandably so because even scientists don’t know a lot about this topic—and will try to make it as easy as possible to understand.

Can you guess what the topic is?

If you guessed dark matter, then you’re correct! So without further ado, let us plunge into the light-bending gravity-filling world of this fascinating entity.

Before we discuss what dark matter exactly is, we should discuss how dark matter was found in the first place. Let’s take a look at this graph:

 Do Elliptical Galaxies Have Dark Matter Halos? | astrobites

Credit: Astrobites

This is known as a galaxy rotation curve and is essentially a plot of a star’s orbital velocity based on their distance from the center of the galaxy.

Now, based on what they could see, astronomers plotted the predicted data of stars’ velocities and ended up with a line looking much like A on the graph. But when scientists actually looked at the stars’ velocities and plotted the data, they ended up with a line looking more like B.

This confused astronomers considerably. 

As you can see, the lines for A and B are quite far apart. For you math nerds out there, you can also observe that the end behavior of both lines is also considerably different. But why was this happening?

The answer, it turns out, is dark matter.

Observing this enormous difference in rotation curves, astronomers hypothesized that in order to account for it, the mass of the galaxy has to be more than observed. As a result, there had to be some matter in the galaxy that was previously unaccounted for.

Astronomers then started to try to break down what the matter could be. They knew some things about it; it had mass, which meant that it could interact with gravity; it did not interact with anything on the electromagnetic spectrum; it did not give off any light at all. Therefore, the term “matière obscure” was coined, which translates from French to “dark matter”.

The evidence for dark matter started to pile up. Astronomers started to notice strong gravitational lensing or the bending of light due to strong gravity. Here is an example of gravitational lensing:

 Weak gravitational lensing - Wikipedia

Credit: NASA

As you can see, the strong gravity of the central object distorts the light coming from the galaxy in the background around it.

Essentially, by studying this dispersion pattern of the light around the object, astronomers could do some fancy math to determine the mass of the object creating the effect of the light bending. And once again, some more mass than was visible was causing the bending. More and more evidence was pointing to the existence of this dark matter.

Eventually, scientists boiled down everything that we know about dark matter. Here it is for your reference.

1. It has gravity.
2. It does not interact with light.

Yep, that’s it. One of the greatest modern unsolved problems in physics is where dark matter comes from, what is it made of, and what it even looks like. Computer simulations have been able to model the distribution of dark matter based on gravitational lensing:


Credit: NASA

And scientists believe that this is the distribution of matter in the universe:

 GMS: Content of the Universe Pie Chart

Credit: NASA

As you can see, dark matter (and something mysterious called dark energy which we’ll get to next time) dominates this graph considerably. In fact, the matter that we, our planet, our life is made up of, is a small fraction of the total matter in the universe.

This kind of matter is also called baryonic matter and forms the root of another unsolved problem in physics. But that is a conversation for another time.

The next time you go outside at night, look to the skies. Even if it may not be visible, dark matter will be there in outer space. Who knows? In the future, we may be able to capture a piece of dark matter and bring it back to Earth. And for you students out there; you may be the person to finally solve the problem.

I find dark matter to be one of the clearest symbols of why we still need science. We may have figured out a lot about the world, but we definitely don’t know anything. And there is literally a universe of possibility out there—worlds upon worlds upon worlds waiting to be found.

Clear skies!