Can there be planets, stars and galaxies made of dark matter or antimatter?

  • 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?

    Well, galaxies are mostly dark matter and baryons wouldn’t have been able to condense without it, so in that sense, yes.

    Where did you get the idea that the universe has more antimatter than regular matter? Antimatter is comparatively rare.

    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.

  • Allure

    Allure Correct answer

    one year ago

    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, 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.

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

    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 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."

    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".

    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-ReinstateMonica according to the Wikipedia article on solar wind (, 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).

    @Peter-ReinstateMonica if you are interested in the numbers I am pretty sure there's a section on it in this book (, somewhere around chapter 5 (?). Unfortunately I don't have access to the book anymore, so I can't pull up the relevant quotes.

License under CC-BY-SA with attribution

Content dated before 7/24/2021 11:53 AM