Are There Dark Matter Galaxies? ft. Sarah Pearson from Space with Sarah

Are There Dark Matter Galaxies? ft. Sarah Pearson from Space with Sarah

We know there’s dark matter, and there are galaxies, but are there galaxies entirely made up of dark matter? Astronomer Sarah Pearson joins Fraser to talk about what’s out there.

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Team: Fraser Cain – @fcain /
Karla Thompson – @karlaii
Chad Weber –

One of the things I love about astronomy is how it’s rapidly changing and evolving over time. Every day there are new discoveries, and advancements in theories that take us incrementally forward in our understanding of the Universe.

One of the best examples of this is dark matter; mysterious and invisible but a significant part of the Universe and accounting for the vast majority of mass out there.

It was first theorized almost 100 years ago when astronomers surveyed the total mass of distant galaxy clusters and found that the visible mass we can see must be just a fraction of the total material in the clusters. When you add up the stars and gas, galaxies move and rotate in ways that indicate there’s a huge halo of invisible matter surrounding it.

Some of the best evidence came from Vera Rubin and Kent Ford in the 60s and 70s, when they measured the rotational velocity of edge-on spiral galaxies. They estimated that there must be about 6 times as much dark matter as regular matter.

Dark matter became a serious mystery in astronomy, and many observers and theorists have spent the last half century trying to work out what it is.

And dark matter hasn’t given up its secrets easily. Originally, astronomers thought it might not actually be invisible mass, but a misunderstanding of how gravity works at the largest scales.

But over the last few decades, techniques have been developed, using the gravity of dark matter itself to measure how it bends light from more distant objects. Astronomers don’t know what dark matter is, but they’re able to use it as a telescope. Now that’s impressive.

They’ve found amazing features in the dark matter web out there, vast walls and filaments defining the largest scale structures in the Universe. Clusters where dark matter and its gas have been separated from each other.

Remember, we are at the cutting edge of this mystery, and you’re watching it unfold in real time. 25 years from now, I’m sure we’ll look back at our quaint attempts to understand dark matter.

One of the most interesting questions I have right now is: could there be dark matter galaxies? Completely invisible to our eyes, but able to interact through gravity?

Of course, in times like this, I like to bring in a ringer. Someone who has dedicated their life to the study of these questions.

And today, I’ve got with my Sarah Pearson, a graduate student in astronomy at Columbia University and the host of “Space with Sarah”. Sarah studies the formation and interactions of dwarf galaxies surrounding the Milky Way to understand how galaxies built up at the earliest times in the Universe and form the large galaxies we see at present day.

Sarah, welcome to the Guide to Space:

Sarah: Hi Fraser, thanks.

Fraser: Can you talk a little bit about how astronomers map out the distribution of dark matter in the Universe?

Sarah: Yes, definitely. So that is a hard question, as you just explained, we don’t see the dark matter. But one assumption about the Universe we live in is that the light matter or baryonic matter. For example, what you, me and stars consist of, and also galaxies, kind of trace out where the dark matter is located.

So one assumption is that the light matter follows the dark matter. In that way we can actually map out to huge distances, kind of how galaxies and clusters of galaxies are located in our Universe. And we imagine that the dark matter structure is somewhat similar.

And also recently, very large scale structure simulations of our own Universe have addressed this by kind of starting out with an almost uniform distribution of dark matter in the very early Universe. And what they see is when they let the Universe evolve in time, for example, when the Universe is expanding, you kind of have these dark matter clumps forming into galaxies in all these filaments that you discussed.

You can kind of trace out the location of dark matter by understanding the expansion of space versus gravity that creates the galaxies that we see.

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