How do we know how much dark matter there is in the Universe?

00:00 Introduction
02:04 Ground News AD
03:54 Method 1 - Galaxy Clusters and the virial theorem
08:49 Method 2 - Big Bang Nucleosynthesis
11:39 Method 3 - Cosmic Microwave Background
14:35 Outro
15:24 Bloopers

Video filmed on a Sony ⍺7 IV

---

---

---

---

---

🔔 Don't forget to subscribe and click the little bell icon to be notified when I post a new video!

---

👩🏽‍💻 I'm Dr. Becky Smethurst, an astrophysicist at the University of Oxford (Christ Church). I love making videos about science with an unnatural level of enthusiasm. I like to focus on how we know things, not just what we know. And especially, the things we still don't know. If you've ever wondered about something in space and couldn't find an answer online - you can ask me! My day job is to do research into how supermassive black holes can affect the galaxies that they live in. In particular, I look at whether the energy output from the disk of material orbiting around a growing supermassive black hole can stop a galaxy from forming stars.

[lethargogpeterson]

Ooo, missing baryon problem video please :-) (When convenient.)

[ericeaton]

An episode covering "the energy budget of the universe" would also be a great addition! You mentioned it several times in this video, but simply as a given (I know it's not what this video was about, haha).

[jml_]

Fun fact, Hans Bethe didn't contribute to the original paper. George Gamow just thought it would be fun to have the authors names be the first three Greek letters.

[threadripper]

Now when people ask, "What's the matter?" I'll have an answer.

[ayac.]

Thanks for fixing the echo issue. Your previous video was also a bit echoey but I didn't say anything to not seem annoying lol. This is the first video that you've truly fixed it so I just wanted to thank you for it.🎉

[SiqueScarface]

15:45 I have to defend Fritz Zwicky here. In his 1936 paper, he clearly states that he only calculates with stars bright enough so we can see them. He does not speculate about the nature of all the stuff we can't see, just that it does not send out enough light for us to detect it. Hence he calls it "Dark Matter", because to him, that's what it was, matter not bright enough to be seen from the Earth. He also extensively quotes Edwin Hubble and his observations of the expanding universe, and because methods at the time weren't well calibrated, he uses a number for the Hubble constant that is way off today's estimates: 500 km/(s*Mpc), seven times higher than currently measured - but that's the number all cosmologists at the time, including Albert Einstein and Alexander Friedmann, were using.

[asterialumin_]

Hello there, Dr Becky. Ive watched many of your videos and I must say you are the best astrophysicist YouTuber I know. The detail, the information and the way you turn complex concepts, easier to understand, is absolutely amazing! I wanted to ask you if you could make a video on how we heard the first sound after the big bang and how it was decoded for humans to hear. It sounds like a pretty interesting topic and id love to hear your input on it.
Hoping you'll consider my request. Thank you. Also, i just received your book 'the brief history of Black holes' and its really beautiful and looks so interesting. Excited to read!

And happiest birthday! Wishing you an amazing life full of new discoveries and successes ahead!

[samuilzaychev]

Love how half of this comment section is conspiracy theories an dthe other half is people saying becky is amazing😂 though this is said near the upload of the video so it will probably change

[Backpack-suyn]

I purchased your "Brief History" book. Even though it has many equations I don't fully understand, I found it very informative. I actually read it twice. The second time, i was able to understand even more. Excellent read...

[daviddenaldi]

Thanks for the video- still my favorite topic in astronomy and physics.

[dominikvonlavante]

Becky. This was amazing. Can you please do more of these basic lectures.

[flwi]

So fascinating how clever minds come up with all these conclusions. The more I learn about the universe the more in awe I look at the night sky. Thanks for sharing your knowledge and passion!

[patchvonbraun]

We're running our first STEM summer camp at the observatory this summer. The main task of the students will be to make observations of the galactic hydrogen, in the first quadrant of the galactic plane, and use that to show that the rotation curve is non-Newtonian, and thus supports the Dark Matter hypothesis.

[BeckyStern]

I learn so much from your channel, oh dear Dr. Becky!!! Thanks for explaining. 🥰

[KipIngram]

Yeah, I think if we can assess this in three ways that are truly independent of each other and arrive at the same estimate, to within a reasonable range, that's pretty compelling.

[stephenmiller]

I watched a PBS show yesterday where they talked about neutrinos and how the elusive "fourth" neutrino may lead to a way to actually identify and measure dark mater. Can you shed some light on this? I may be thoroughly wet with how I interpreted what I heard, but I thought I heard about some relationship between dark matter and this theorized, yet to be actually discovered fourth neutrino.

[longlostkryptonian]

For all the trolls out there, just because some things are beyond your comprehension, doesn’t mean they’re beyond everyone’s or that they’re not a possible part of reality.

[conrad]

This was really cool. I'd only ever heard of the first way. Fascinating to learn about the other two. Thank you.

[kasperholm]

the night betwen the 8th and 9th of may 2024, there was a beautifull night sky with many stars really shinning, i looked up that night. Where i live it gets pretty dark compared to places like big cities, or areas around places like that, and we have those LED streetlamps that turns way down when there is not traffic. And for the first time in like 8 months there where no clouds, no moon, it was warm enough to be outside without it being to cold, and there where no wind. I spend a good part of an hour just looking up, and more and more stars got visiable. At one point my gaze just slide over the sky i saw the big dipper, and i simply just couldnt stop starring at the general area of where hubble snapped the deep field image. 1000s of galaxies in that tiny spot and my thought sweept past Carl Sagans "billions and billions of stars".. I then searched and found the andromeda galaxy a tad under the W, i had to put my glasses on for that one though. But just seing it and thinking that light was emitted roughly the same time as the early species of humanoids where learning to use stone tools, and now i see the photons is mesmerising. I could keep going but i just want to say our night sky is so beautiful and awesome to look at when its visable.

[curtislindsey]

I think it means we've figured out about 5% of the universe. It's not a bad start but we clearly have a lot to figure out.