<p>Probably not. Dark matter should really be called &quot;transparent matter&quot; since it does not interact with light. This has an important consequence: it is hard for dark matter - whatever it is - to lose energy by radiating. This is why normal matter can form clouds that accrete into dense regions that in turn become galaxies and stars: energy is radiated away. But dark matter cannot do this as far as we know, so instead it forms large diffuse &quot;halos&quot; that surround galaxies.</p>
<p>Antimatter is completely different from dark matter. For some reason (important research topic) there is far more normal matter than antimatter in the universe, and all primordial antimatter is likely to have reacted with the matter in the early eras. Hence there are not going to be enough of it to form antiplanets, stars or galaxies.</p>

CC BY-SA 4.0
2021-01-03T22:39:04.987Z
Does this limitation on condensed anti-matter only apply to the visible universe, or is there some reason to believe that the preference for matter over anti-matter is somehow fundamental?
SoronelHaetir
CC BY-SA 4.0
2021-01-03T22:55:01.240Z
@SoronelHaetir As I understand things, we would expect to see radiation coming off of the boundary between matter-rich and antimatter-rich regions if any of the latter exist, and the presence of that radiation, even outside the observable universe, would have affected how the universe evolved to its current form in ways we don't see.
Hearth
CC BY-SA 4.0
2021-01-04T12:49:15.530Z
Strictly speaking, all we know is that the area we can observe within the universe has an overabundance of normal matter relative to antimatter, not that the whole universe does, because we do not know if the observable universe is the whole universe or some small part of it.
Austin Hemmelgarn
CC BY-SA 4.0
2021-01-04T13:28:50.350Z
@Austin We know that the whole universe is larger than the observable universe, but we don't know by how much. It's (probably) at least 150× bigger, and there are good reasons to believe it's actually infinite, although (of course) we'll never be able to verify that by direct measurement. Please see https://astronomy.stackexchange.com/a/31795/16685 for further details.
PM 2Ring
CC BY-SA 4.0
2021-01-05T09:53:32.743Z
@AustinHemmelgarn would a matter - antimatter segregation at some larger than obs Un be theorical possible without affecting the cosmological principle? I am tempted to say yes, at least from the point at which segregation is in place.
Alchimista
CC BY-SA 4.0
2021-01-03T12:24:11.733Z
2021-01-03T12:24:11.733Z
Anders Sandberg
<p>Gravitational lensing observations suggest that there is a large mass of dark matter on either side of the <a href="https://en.wikipedia.org/wiki/Bullet_Cluster" rel="nofollow noreferrer">bullet cluster</a>, which is actually one of the major pieces of evidence that dark matter does indeed exist. This dark matter essentially &quot;left behind&quot; the majority of the normal matter in the galaxies it was with as two galaxy clusters collided and most of the normal matter in them got tangled up in the middle. These globs of dark matter with little normal matter probably could, if you like, be considered (in a non-technical sense) to be dark matter galaxies. They're not 100% dark matter, as most of the galaxies' stars also went with them, but they are, at least as I understand it, more dark matter than not.</p>
<p>This isn't the only such object; a similar collision between galaxy clusters produced the object <a href="https://en.wikipedia.org/wiki/MACS_J0025.4-1222" rel="nofollow noreferrer">MACS J0025.4-1222</a>, which also consists of several galaxies worth of dust and gas stripped of their dark matter with a pile of dark matter and stars on either side.</p>

CC BY-SA 4.0
2021-01-03T23:02:35.413Z
2021-01-04T13:25:42.913Z
2021-01-04T13:25:42.913Z
Hearth
<p><strong>Dark matter galaxies</strong> are possible but very speculative. On a theoretical level, they are hard to form because dark matter interacts only gravitationally (see Anders Sandberg's answer), which makes it hard to lose energy and become bound structures. On an observational level, they would be hard to detect. Gravitational lensing can do something, but since one cannot actually see the galaxy, it's also hard to say where the dark galaxy is -- if there is one at all.</p>
<p>Still, people have <a href="https://en.wikipedia.org/wiki/Dark_galaxy" rel="noreferrer">studied the idea</a>, so it's not impossible.</p>
<p><strong>Antimatter galaxies</strong>: At some level the idea that there are antimatter galaxies out here is appealing. First it can solve the baryon asymmetry problem at a stroke. It's also the case that an antimatter star would shine. From long distance, it would also be practically indistinguishable from a &quot;normal&quot; star.</p>
<p><em>However</em>, there are strong reasons to believe that there are no antimatter galaxies. That's because antimatter annihilates with normal matter, which leaves experimental signatures. If any part of the Earth were made of antimatter, it would immediately vanish in a flash, so we can be sure that the Earth is mostly matter. Similarly, if the Sun were made of antimatter, we would be quickly annihilated (thanks to the antimatter solar wind radiating from the anti-Sun), so we can be sure the Sun is also mostly matter. Similar arguments allow us to conclude that the Milky Way is almost entirely matter, the Local Group is almost entirely matter, etc, <a href="https://en.wikipedia.org/wiki/Baryon_asymmetry#Regions_of_the_universe_where_antimatter_dominates" rel="noreferrer">all the way up to the largest structures in the sky</a>.</p>
<p>If antimatter galaxies exist, they are probably outside our observable universe, at which point some will argue it's no longer science.</p>

CC BY-SA 4.0
2021-01-06T14:27:26.063Z
What's the mass flux from solar wind hitting Earth? I suppose one would have measurable gamma radiation if it were antimatter but I doubt it's enough to "annihilate" us, let alone "quickly".
Peter - Reinstate Monica
CC BY-SA 4.0
2021-01-06T14:57:14.400Z
Hmmm... a quick estimate is 3E16 W from 1 kg/s hitting Earth. Total irradiation is apparently 1.7E17 W already. So no annihilation but a significant change in the energy budget. And it should indeed be easily recognizable.
Peter - Reinstate Monica
CC BY-SA 4.0
2021-01-06T23:24:08.170Z
@Peter-ReinstateMonica according to the Wikipedia article on solar wind (https://en.wikipedia.org/wiki/Solar_wind#Acceleration), the Sun loses a million tons a second due to the wind, and radiates an Earth mass every ~150 million years. This will definitely annihilate us "quickly" (in astronomy, 150 million years is not a long time at all).
Allure
CC BY-SA 4.0
2021-01-06T23:37:17.400Z
@Peter-ReinstateMonica if you are interested in the numbers I am pretty sure there's a section on it in this book (https://www.amazon.com/Story-Antimatter-Matters-Vanished-Twin-ebook/dp/B07D2YMHTB), somewhere around chapter 5 (?). Unfortunately I don't have access to the book anymore, so I can't pull up the relevant quotes.
Allure
CC BY-SA 4.0
2021-01-05T13:38:16.490Z
Why would the "we would be quickly annihilated" argument also work for galaxies? I get it that inside the Solar System space is not empty enough and if there was antimatter it would interact... similarly for stuff inside our galaxy. But galaxies are very far from each other, so why would that argument automatically apply to them as well, if one galaxy was made entirely out of matter, and the other one entirely out of antimatter, with millions of lightyears between them. And if that's also too close, then what about galaxy clusters?
vsz
CC BY-SA 4.0
2021-01-05T19:21:08.990Z
@vsz check the link - "The density of matter in intergalactic space is reasonably well established at about one atom per cubic meter. Assuming this is a typical density near a boundary, the gamma ray luminosity of the boundary interaction zone can be calculated. No such zones have been detected, but 30 years of research have placed bounds on how far they might be."
Allure
CC BY-SA 4.0
2024-12-30T20:17:17.780Z
@Allure It sounds like a lot, but the Sun radiates that in all directions. By a [rough calculation](https://www.wolframalpha.com/input?i=%28%28pi%29%28radius+of+the+earth%29%5E2%29+%2F+%284+pi+%281+au%29%5E2%29+*+1.5+million+tonnes) the Earth only receives 5e-10 of that or less than 1kg per second.
Schwern
CC BY-SA 4.0
2024-12-31T00:31:36.827Z
@Schwern I'm no expert on this, and I haven't done the calculations. However I definitely remember reading that being a planet of an anti-sun would have fatal consequences. Currently the Earth gains mass from meteorites and loses it to outgassing. With an anti-sun, there would be fewer meteorites which would probably have an impact. Furthermore, as the Earth loses mass, its gravity would also weaken, which means even more outgassing / fewer meteorites. But again, I've not done the math.
Allure
CC BY-SA 4.0
2021-01-04T05:28:26.743Z
2021-01-05T00:26:25.740Z
2021-01-05T00:26:25.740Z
Allure
<p>We know that the universe has more dark and anti matter as compared to normal matter. Can there be dark matter galaxies or antimatter galaxies?</p>

CC BY-SA 4.0
2021-01-03T23:17:37.950Z
Well, galaxies are mostly dark matter and baryons wouldn’t have been able to condense without it, so in that sense, yes.
pela
CC BY-SA 4.0
2021-01-04T13:30:23.953Z
Where did you get the idea that the universe has more antimatter than regular matter? Antimatter is comparatively rare.
PM 2Ring
CC BY-SA 4.0
2021-01-04T18:56:43.470Z
In theory, there *could* be antimatter galaxies out there -- they wouldn't annihilate with regular matter because of the *spacing* between galaxies -- they just wouldn't come in contact.  However, according to Paul LaViolette's Model G Theory, antimatter is very rare and unstable because regular protons have long lives before decaying, while antiprotons have very short lives.
Jennifer
CC BY-SA 4.0
2021-01-05T15:54:57.127Z
Mandatory XKCD: [What if everything was antimatter, EXCEPT Earth?](https://what-if.xkcd.com/114/)
Geeky Guy
<universe><dark-matter><antimatter>
Can there be planets, stars and galaxies made of dark matter or antimatter?
Dark Knight
# Can there be planets, stars and galaxies made of dark matter or antimatter?

(Asked by: username_0 on 2021-01-03)

We know that the universe has more dark and anti matter as compared to normal matter. Can there be dark matter galaxies or antimatter galaxies?

**Tags:** universe, dark-matter, antimatter
> **Comments:**
> **username_1**: Well, galaxies are mostly dark matter and baryons wouldn’t have been able to condense without it, so in that sense, yes.  
> **username_2**: Where did you get the idea that the universe has more antimatter than regular matter? Antimatter is comparatively rare.  
> **username_3**: In theory, there *could* be antimatter galaxies out there -- they wouldn't annihilate with regular matter because of the *spacing* between galaxies -- they just wouldn't come in contact.  However, according to Paul LaViolette's Model G Theory, antimatter is very rare and unstable because regular protons have long lives before decaying, while antiprotons have very short lives.  
> **username_4**: Mandatory XKCD: [What if everything was antimatter, EXCEPT Earth?](https://what-if.xkcd.com/114/)  

---

## 3 answers

### Answer by username_10 on 2021-01-04. Score: 31 | Accepted answer

**Dark matter galaxies** are possible but very speculative. On a theoretical level, they are hard to form because dark matter interacts only gravitationally (see Anders Sandberg's answer), which makes it hard to lose energy and become bound structures. On an observational level, they would be hard to detect. Gravitational lensing can do something, but since one cannot actually see the galaxy, it's also hard to say where the dark galaxy is -- if there is one at all.

Still, people have [studied the idea](https://en.wikipedia.org/wiki/Dark_galaxy), so it's not impossible.

**Antimatter galaxies**: At some level the idea that there are antimatter galaxies out here is appealing. First it can solve the baryon asymmetry problem at a stroke. It's also the case that an antimatter star would shine. From long distance, it would also be practically indistinguishable from a "normal" star.

*However*, there are strong reasons to believe that there are no antimatter galaxies. That's because antimatter annihilates with normal matter, which leaves experimental signatures. If any part of the Earth were made of antimatter, it would immediately vanish in a flash, so we can be sure that the Earth is mostly matter. Similarly, if the Sun were made of antimatter, we would be quickly annihilated (thanks to the antimatter solar wind radiating from the anti-Sun), so we can be sure the Sun is also mostly matter. Similar arguments allow us to conclude that the Milky Way is almost entirely matter, the Local Group is almost entirely matter, etc, [all the way up to the largest structures in the sky](https://en.wikipedia.org/wiki/Baryon_asymmetry#Regions_of_the_universe_where_antimatter_dominates).

If antimatter galaxies exist, they are probably outside our observable universe, at which point some will argue it's no longer science.

> **Comments:**
> **username_12**: Why would the "we would be quickly annihilated" argument also work for galaxies? I get it that inside the Solar System space is not empty enough and if there was antimatter it would interact... similarly for stuff inside our galaxy. But galaxies are very far from each other, so why would that argument automatically apply to them as well, if one galaxy was made entirely out of matter, and the other one entirely out of antimatter, with millions of lightyears between them. And if that's also too close, then what about galaxy clusters?  
> **username_10**: @vsz check the link - "The density of matter in intergalactic space is reasonably well established at about one atom per cubic meter. Assuming this is a typical density near a boundary, the gamma ray luminosity of the boundary interaction zone can be calculated. No such zones have been detected, but 30 years of research have placed bounds on how far they might be."  
> **username_11**: What's the mass flux from solar wind hitting Earth? I suppose one would have measurable gamma radiation if it were antimatter but I doubt it's enough to "annihilate" us, let alone "quickly".  
> **username_11**: Hmmm... a quick estimate is 3E16 W from 1 kg/s hitting Earth. Total irradiation is apparently 1.7E17 W already. So no annihilation but a significant change in the energy budget. And it should indeed be easily recognizable.  
> **username_10**: @Peter-ReinstateMonica according to the Wikipedia article on solar wind (https://en.wikipedia.org/wiki/Solar_wind#Acceleration), the Sun loses a million tons a second due to the wind, and radiates an Earth mass every ~150 million years. This will definitely annihilate us "quickly" (in astronomy, 150 million years is not a long time at all).  
> **username_10**: @Peter-ReinstateMonica if you are interested in the numbers I am pretty sure there's a section on it in this book (https://www.amazon.com/Story-Antimatter-Matters-Vanished-Twin-ebook/dp/B07D2YMHTB), somewhere around chapter 5 (?). Unfortunately I don't have access to the book anymore, so I can't pull up the relevant quotes.  
> **username_13**: @Allure It sounds like a lot, but the Sun radiates that in all directions. By a [rough calculation](https://www.wolframalpha.com/input?i=%28%28pi%29%28radius+of+the+earth%29%5E2%29+%2F+%284+pi+%281+au%29%5E2%29+*+1.5+million+tonnes) the Earth only receives 5e-10 of that or less than 1kg per second.  
> **username_10**: @Schwern I'm no expert on this, and I haven't done the calculations. However I definitely remember reading that being a planet of an anti-sun would have fatal consequences. Currently the Earth gains mass from meteorites and loses it to outgassing. With an anti-sun, there would be fewer meteorites which would probably have an impact. Furthermore, as the Earth loses mass, its gravity would also weaken, which means even more outgassing / fewer meteorites. But again, I've not done the math.  

---

### Answer by username_5 on 2021-01-03. Score: 29

Probably not. Dark matter should really be called "transparent matter" since it does not interact with light. This has an important consequence: it is hard for dark matter - whatever it is - to lose energy by radiating. This is why normal matter can form clouds that accrete into dense regions that in turn become galaxies and stars: energy is radiated away. But dark matter cannot do this as far as we know, so instead it forms large diffuse "halos" that surround galaxies.

Antimatter is completely different from dark matter. For some reason (important research topic) there is far more normal matter than antimatter in the universe, and all primordial antimatter is likely to have reacted with the matter in the early eras. Hence there are not going to be enough of it to form antiplanets, stars or galaxies.

> **Comments:**
> **username_6**: Does this limitation on condensed anti-matter only apply to the visible universe, or is there some reason to believe that the preference for matter over anti-matter is somehow fundamental?  
> **username_7**: @SoronelHaetir As I understand things, we would expect to see radiation coming off of the boundary between matter-rich and antimatter-rich regions if any of the latter exist, and the presence of that radiation, even outside the observable universe, would have affected how the universe evolved to its current form in ways we don't see.  
> **username_8**: Strictly speaking, all we know is that the area we can observe within the universe has an overabundance of normal matter relative to antimatter, not that the whole universe does, because we do not know if the observable universe is the whole universe or some small part of it.  
> **username_2**: @Austin We know that the whole universe is larger than the observable universe, but we don't know by how much. It's (probably) at least 150× bigger, and there are good reasons to believe it's actually infinite, although (of course) we'll never be able to verify that by direct measurement. Please see https://astronomy.stackexchange.com/a/31795/16685 for further details.  
> **username_9**: @AustinHemmelgarn would a matter - antimatter segregation at some larger than obs Un be theorical possible without affecting the cosmological principle? I am tempted to say yes, at least from the point at which segregation is in place.  

---

### Answer by username_7 on 2021-01-03. Score: 5

Gravitational lensing observations suggest that there is a large mass of dark matter on either side of the [bullet cluster](https://en.wikipedia.org/wiki/Bullet_Cluster), which is actually one of the major pieces of evidence that dark matter does indeed exist. This dark matter essentially "left behind" the majority of the normal matter in the galaxies it was with as two galaxy clusters collided and most of the normal matter in them got tangled up in the middle. These globs of dark matter with little normal matter probably could, if you like, be considered (in a non-technical sense) to be dark matter galaxies. They're not 100% dark matter, as most of the galaxies' stars also went with them, but they are, at least as I understand it, more dark matter than not.

This isn't the only such object; a similar collision between galaxy clusters produced the object [MACS J0025.4-1222](https://en.wikipedia.org/wiki/MACS_J0025.4-1222), which also consists of several galaxies worth of dust and gas stripped of their dark matter with a pile of dark matter and stars on either side.


---

