Starts With A Bang!

Dark Matter: More Irrefutable Evidence

A lot of people don’t like dark matter. It’s a crazy idea, after all. Think about everything in your experience: the skies, the sea, you and me, along with the Earth, the Moon, the Sun, and everything we see.

It’s really beautiful, isn’t it? And yet dark matter tells us that for every atom of normal matter in the entire Universe, there is five times as much dark matter, or matter that isn’t made up of protons, neutrons, and electrons.

How do we possibly know this? Well, all matter has something in common: gravity. No matter what you’re made of, if you have mass, you exert a gravitational force on everything else in the Universe. But if you’re made up of normal matter (remember: protons, neutrons, and electrons), you can also emit light under the right conditions. The Sun does this, for example.

So how can we find dark matter? Let’s take the biggest things in the Universe: clusters of galaxies. These are regions of space that have hundreds or even thousands of galaxies the size of our Milky Way in them, and all told they weigh over a quadrillion times as much as our Sun. It would make a great experiment if we could smash two of them together. Because the normal matter should stick together and heat up, and emit X-rays. But if there’s some other type of matter, something different from normal matter, it should just pass right through everything, right on through the other galaxy cluster, like an object in motion remaining in motion. So we look for two clusters of galaxies colliding:

Ladies and gentlemen — BEHOLD! I give you exhibit A, known as the Bullet Cluster. This is two clusters of galaxies colliding, and you can see the individual galaxies here. The pink is the X-ray gas, and you can see that’s where the normal matter collided, stuck together, and emits powerful light in the form of X-rays. But the blue, that’s where all the mass is. From a phenomenon called gravitational lensing, we can measure how much mass there is in a certain region of space. And as you can see, we find that most of the mass is not where the normal matter is.

But we are responsible people, us scientists, and we like to have more than one example before we draw a conclusion. And so I offer you exhibit B, cluster Abell 520:

BEHOLD! This is a cluster in the later stages of merger, so that some of the dark matter has had a chance to come back around to their mutual center of mass. Still, the light coming from the normal matter doesn’t trace where the mass is.

So we’re getting close, but can we find an example of this far away? Can we find a very distant cluster that has these same properties? Let’s take a look at a very special cluster: MACS J0025.

Looks like a big honking cluster, and it contains over 1,000 galaxies and weighs in at over a quadrillion solar masses. A big one, for sure. But where is all the mass? Let’s take a look at the gravitational lensing data and see (in blue false color) where it is:

Ahh, it looks like just two lobes, one towards the left of the image and one towards the right. Well, if these were two clusters that just collided, that would be all the matter that just passed through the center. Except normal matter doesn’t do that! It collides, sticks together, and heats up. What if we took our big X-ray observatory, Chandra, and looked at this cluster. Would we see X-rays coming from the middle? Where there’s very little mass?

ha-HA, we do! So when we put all these together, what do we find? Could the normal matter explain all the gravity we see, or do we need something else; some new type of matter? Ladies and gentlemen:

BEHOLD!!! It’s dark matter, different from normal matter, not made up of protons, neutrons, and electrons, but full of mass and exerting a gravitational force nonetheless. There are some awesome animations up at the Chandra Website, which I highly recommend to you.