5 Replaced arXiv link with journal link as per https://skeptics.meta.stackexchange.com/a/1387/22247.
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Data from EGRET (Reimer et al. 2003) constrains the ratio of the dominant component (either matter or antimatter) to the rarer component (either antimatter or matter) in a number of prominent clusters (Steigman 2008; Steigman 2008arXiv version). For many, e.g. the Virgo Cluster and the Perseus Cluster, the fraction is around ~ 10-8 or lower. An outlier is the Bullet Cluster, which has a comparatively high ~ 10-5.5, but most clusters are well below this.

Data from EGRET (Reimer et al. 2003) constrains the ratio of the dominant component (either matter or antimatter) to the rarer component (either antimatter or matter) in a number of prominent clusters (Steigman 2008). For many, e.g. the Virgo Cluster and the Perseus Cluster, the fraction is around ~ 10-8 or lower. An outlier is the Bullet Cluster, which has a comparatively high ~ 10-5.5, but most clusters are well below this.

Data from EGRET (Reimer et al. 2003) constrains the ratio of the dominant component (either matter or antimatter) to the rarer component (either antimatter or matter) in a number of prominent clusters (Steigman 2008; arXiv version). For many, e.g. the Virgo Cluster and the Perseus Cluster, the fraction is around ~ 10-8 or lower. An outlier is the Bullet Cluster, which has a comparatively high ~ 10-5.5, but most clusters are well below this.

4 :-/ I can't type this week, it seems; corrected cluster fraction. Also discussed antimatter stars.
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The answer to your questions is complicated. We know that in the Milky Way, and in the Local Group, matter must dominate antimatter by 10-15 orders of magnitude. We've also put limits on other galaxy clusters that indicate that the ratio is anywhere from about 5.5 to 8 orders of magnitude, depending on the cluster. We don't know whether the dominant component of these clusters is matter or antimatter; we just know that there's a very high asymmetry, and the consensus, applying the cosmological principle, is that it's more likely to be matter that dominates, rather than antimatter. If we could show that the dominant component in those clusters is matter, the answer to your question would be a firm yes.

In the interstellar medium (ISM) between stars, we can try to find pockets of antimatter (Steigman 1976). One simple direct detection method involves observing the polarization of light passing through the ISM and an effect called Faraday rotation, where light interacts with a large-scale magnetic field. Large quantities of antimatter (likely antiprotons and positrons) should leave a characteristic polarization signature on this rotation, but this has not been observed even since the first studies (Gardner & Whiteoak 1963 onwards). Indirect ISM measurements of the products of matter-antimatter annihilation (gamma rays, chiefly, as well as neutrinos) have very strongly constrained upper limits to galactic antimatter fractions:

Steigman's review also mentions that only a few compact gamma ray sources have been found, implying a lack of stars made of antimatter, but this is quite out of date, as both Fermi and more modern gamma ray telescopes have found many more high-energy sources (see e.g. TeVCat for a list of high-energy galactic and extragalactic sources). That said, we've identified the nature of these sources (pulsar wind nebulae, high energy binaries, etc.), and antimatter stars can still likely be ruled out.

The upper bound on the fraction in the galaxy placed by Steigman is ~ 10-4, corresponding to about 10 million stars. This assumes that the stars were formed from condense pockets of antimatter, as the lifetime of an antiparticle in the ISM is only about 300 years. The calculation was done based on the expected luminosity from matter-antimatter annihilation as a star passes through gas, and compared to the Milky Way's total gamma ray luminosity.

The answer to your questions is complicated. We know that in the Milky Way, and in the Local Group, matter must dominate antimatter by 10-15 orders of magnitude. We've also put limits on other galaxy clusters that indicate that the ratio is anywhere from about 5.5 to 8, depending on the cluster. We don't know whether the dominant component of these clusters is matter or antimatter; we just know that there's a very high asymmetry, and the consensus, applying the cosmological principle, is that it's more likely to be matter that dominates, rather than antimatter. If we could show that the dominant component in those clusters is matter, the answer to your question would be a firm yes.

In the interstellar medium (ISM) between stars, we can try to find pockets of antimatter. One simple direct detection method involves observing the polarization of light passing through the ISM and an effect called Faraday rotation, where light interacts with a large-scale magnetic field. Large quantities of antimatter (likely antiprotons and positrons) should leave a characteristic polarization signature on this rotation, but this has not been observed even since the first studies (Gardner & Whiteoak 1963 onwards). Indirect ISM measurements of the products of matter-antimatter annihilation (gamma rays, chiefly, as well as neutrinos) have very strongly constrained upper limits to galactic antimatter fractions:

Steigman's review also mentions that only a few compact gamma ray sources have been found, implying a lack of stars made of antimatter, but this is quite out of date, as both Fermi and more modern gamma ray telescopes have found many more high-energy sources (see e.g. TeVCat for a list of high-energy galactic and extragalactic sources). That said, we've identified the nature of these sources (pulsar wind nebulae, high energy binaries, etc.), and antimatter stars can still be ruled out.

The answer to your questions is complicated. We know that in the Milky Way, and in the Local Group, matter must dominate antimatter by 10-15 orders of magnitude. We've also put limits on other galaxy clusters that indicate that the ratio is anywhere from about 5.5 to 8 orders of magnitude, depending on the cluster. We don't know whether the dominant component of these clusters is matter or antimatter; we just know that there's a very high asymmetry, and the consensus, applying the cosmological principle, is that it's more likely to be matter that dominates, rather than antimatter. If we could show that the dominant component in those clusters is matter, the answer to your question would be a firm yes.

In the interstellar medium (ISM) between stars, we can try to find pockets of antimatter (Steigman 1976). One simple direct detection method involves observing the polarization of light passing through the ISM and an effect called Faraday rotation, where light interacts with a large-scale magnetic field. Large quantities of antimatter (likely antiprotons and positrons) should leave a characteristic polarization signature on this rotation, but this has not been observed even since the first studies (Gardner & Whiteoak 1963 onwards). Indirect ISM measurements of the products of matter-antimatter annihilation (gamma rays, chiefly, as well as neutrinos) have very strongly constrained upper limits to galactic antimatter fractions:

Steigman's review also mentions that only a few compact gamma ray sources have been found, implying a lack of stars made of antimatter, but this is quite out of date, as both Fermi and more modern gamma ray telescopes have found many more high-energy sources (see e.g. TeVCat for a list of high-energy galactic and extragalactic sources). That said, we've identified the nature of these sources (pulsar wind nebulae, high energy binaries, etc.), and antimatter stars can still likely be ruled out.

The upper bound on the fraction in the galaxy placed by Steigman is ~ 10-4, corresponding to about 10 million stars. This assumes that the stars were formed from condense pockets of antimatter, as the lifetime of an antiparticle in the ISM is only about 300 years. The calculation was done based on the expected luminosity from matter-antimatter annihilation as a star passes through gas, and compared to the Milky Way's total gamma ray luminosity.

3 added 724 characters in body
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The answer to your questions is complicated. We know that in the Milky Way, and in the Local Group, matter must dominate antimatter by 10-15 orders of magnitude. We've also put limits on other galaxy clusters that indicate that the ratio is anywhere from about 5.5 to 8, depending on the cluster. We don't know whether the dominant component of these clusters is matter or antimatter; we just know that there's a very high asymmetry, and the consensus, applying the cosmological principle, is that it's more likely to be matter that dominates, rather than antimatter. If we could show that the dominant component in those clusters is matter, the answer to your question would be a firm yes.

In terms of techniques: we can measure the ratio of antimatter to matter on a variety of scales, from inside the galaxy (i.e. a few parsecskiloparsecs) to entire galaxy clusters (on the order of tens of megaparsecs) by searching for the signature of matter-antimatter annihilation - namely, photons - or antiparticles themselves. Over the last half century, stringent limits have been placed on this ratio in different locations and regimes. Very low limits are known for our own galaxy, while for clusters far away, we have results that aren't as low but are still quite good - although we can't prove whether the predominant component in the cluster is matter or antimatter.

We can measure the ratio of antimatter to matter on a variety of scales, from inside the galaxy (i.e. a few parsecs) to entire galaxy clusters (on the order of tens of megaparsecs) by searching for the signature of matter-antimatter annihilation - namely, photons - or antiparticles themselves. Over the last half century, stringent limits have been placed on this ratio in different locations and regimes. Very low limits are known for our own galaxy, while for clusters far away, we have results that aren't as low but are still quite good - although we can't prove whether the predominant component in the cluster is matter or antimatter.

The answer to your questions is complicated. We know that in the Milky Way, and in the Local Group, matter must dominate antimatter by 10-15 orders of magnitude. We've also put limits on other galaxy clusters that indicate that the ratio is anywhere from about 5.5 to 8, depending on the cluster. We don't know whether the dominant component of these clusters is matter or antimatter; we just know that there's a very high asymmetry, and the consensus, applying the cosmological principle, is that it's more likely to be matter that dominates, rather than antimatter. If we could show that the dominant component in those clusters is matter, the answer to your question would be a firm yes.

In terms of techniques: we can measure the ratio of antimatter to matter on a variety of scales, from inside the galaxy (i.e. a few kiloparsecs) to entire galaxy clusters (on the order of tens of megaparsecs) by searching for the signature of matter-antimatter annihilation - namely, photons - or antiparticles themselves. Over the last half century, stringent limits have been placed on this ratio in different locations and regimes. Very low limits are known for our own galaxy, while for clusters far away, we have results that aren't as low but are still quite good - although we can't prove whether the predominant component in the cluster is matter or antimatter.

2 Made some bits clearer and added a better TL;DR.
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