Science Snippet #3

Dark matter might seem like a crazy concept… Crazy enough that many people believe it does not exist! We have not yet been able to detect it. It doesn’t interact with light or the electromagnetic force, so we can’t see it or measure it with electronic equipment (which is too bad, our eyes and our electronics are typically our most useful tool). The only way we have been able to know it exists is through its interactions with gravity. In fact, we’ve been able to tell that there is more dark matter than regular matter (the stuff we can see and touch) in the universe!

The stuff we can see and touch only makes up about 5% of the universe. Dark matter makes just over 25%! The rest is made up of dark energy, which will be a topic for another post.

(Image from https://www.nist.gov/image/matterpie0png)

So how have we “seen” it if we can’t see it? Well, as I mentioned, it interacts through gravity, so we can see it’s effects on the visible matter around it! One of the most famous examples is deep in space in what’s called the Bullet Cluster.

The Bullet Cluster is two clusters of galaxies that collided and moved past each other. Using x-rays, NASA was able to pinpoint where the regular matter is (in this case, mostly hot gas) and indicated it in pink in the above image. And based on the way the mass was moving and where it was concentrated, they were able to calculate where the dark matter is, which is the blue portions of the image. The dark matter sailed right on through the regular matter without interacting, which is why it’s in a different place than the regular matter, which interacted through collisions! (Image from https://science.nasa.gov/asset/webb/bullet-cluster-webb-and-chandra-image/ )

So what do we do? We go underground because it’s less “noisy” from background radiation and build extremely sensitive detectors, and hope that we see a very rare interaction of a dark matter particle colliding with regular matter.

Don’t worry if this seems a little out there, I was suspicious of it at first, too! Let me tell you a story.

Imagine that our galaxy (the Milky Way) is a car driving around the universe. There are other cars (other galaxies) but we can’t see anything else outside the windows. It looks like we’re driving through empty space. Yet for some reason, the highway we’re driving on suddenly curves as if we’re driving around something in our path. The roads form grids and intersections, we see other cars slowing down, swerving, stopping, turning… And yet we don’t see what’s out there.

It turns out that we’re driving through a massive invisible city! We can’t see it, we can’t touch it, we can’t enter the buildings or ride the elevators, but it’s there. And once in a while, if we’re driving in the right place at the right time, a crumbling piece of brick from one of those invisible buildings or a stone from an invisible bridge might glance off of our fender. And if we’re lucky, we might be able to notice it happen and learn something about this invisible city we’re driving through!

In the meantime, if you take a look to your left, you might faintly make out a sign that says:

Welcome to Dark Matter.
Population: The Universe