#9: What is dark matter?

Hello, reader. With only a few weeks of the senior project left, I am going to use this week's blog post to discuss the most important question of all - what is dark matter? I will tackle this question by first defining normal matter & then explaining how dark matter was discovered.

Everything you see or have the ability to see, from the screen you are reading this on to distant stars, is normal matter. This type of matter only makes up about 4% of the universe.

Dark matter, on the other hand, composes approximately 27% of the universe. We have not yet seen dark matter, & we cannot yet detect it. (Many projects are attempting to do so. You can find the page about a research article on one of these attempts here - the article itself was removed.) This leads to another question - if we cannot observe dark matter, how do we know it exists in the first place?

The answer is best explained using an analogy. Suppose you are in a house and want to determine whether there is wind outside. However, all the windows and the doors of the house are locked - now how will you determine whether there is wind? You can do so indirectly - you observe a flag outside blowing in the wind. Similarly, we know dark matter exists not by directly detecting it, but by observing its effects on other objects we can observe. Specifically, observing galaxies allowed us to develop the theory of dark matter.

It is important to note that galaxies are not 'singular' entities. They are spread out, chunky blobs of stars, dust, gas, & more astronomical objects. All of these components rotate around a center of mass - a black hole. This fact is important because the theory of dark matter came about by analyzing the rotation curves of galaxies.

If a galaxy was a solid disk (which is not the case for the galaxies we are analyzing), the velocity of the galaxy would increase linearly with radius. (See the image below.)

In a galaxy in which most of the mass is concentrated at or around the center, the velocity would decrease with the square root of the radius - this behavior is known as Keplerian decline because this is the case in the solar system.

A flat rotation curve - a rotation curve in which velocity is constant over a range of radii - means that the galaxy's mass is increasing linearly with radius.

While the rotation galaxies of galaxies are expected to follow Keplerian decline (see curve A below), they instead are flat (see curve B below) - this fact presents strong evidence for dark matter.

As I begin to think about my presentation, it is apparent that my biggest challenge will be clearly conveying what I have learned to my audience. It had taken me several weeks to completely understand my COSMOS project, and this project was also as complicated, if not more nuanced. This post was an opportunity not only to address an important question that is part of my project, but also to practice explaining a concept that can be difficult to wrap one's head around both concisely and clearly.

While this skill is difficult to perfect, I am determined to find the perfect mix of depth and clarity when presenting my project to others. Over the next few weeks as I continue to wrap up my project, I will explain my findings but also ensure that my posts can be understood by both those with and without previous knowledge of astrophysics.

If you have any questions or comments about my explanation above (particularly what parts were clear & what parts needed more clarity), please leave a comment below. Until this Thursday, reader!

#update #technical