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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 ...


28

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 ...


22

When you say anti-matter, I assume you mean the opposite of what our Galaxy is made of. Obviously you can label what we are made of as matter or anti-matter as you wish, the point is that the universe as we see it seems to be made (mostly) of one of those possibilities. In principle, distant galaxies could be made of anti-matter - all the physics would be ...


5

Gravitational lensing observations suggest that there is a large mass of dark matter on either side of the 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 ...


1

A recent article, Constraints on the antistar fraction in the Solar System neighborhood from the 10-year Fermi Large Area Telescope gamma-ray source catalog, suggests that there might well be stars made of antimatter in our own galaxy. (arXiv preprint). The claim made in this article is that the very signatures ProfRob discussed in his answer have been ...


1

You basically answered your question with your sub-questions. Detection would be through the process of annihilation - either for individual atoms, or for a star hitting an anti-star with an unfathomably high amount of energy. We would definitely observe the latter, if it ocured in our times on a scale of planets or stars. That said, the important phase of ...


1

Matter and antimatter particles are always produced as a pair. This particle physics' process is called pair production. If antimatter is detectable, it means that the antiparticle of a matter-antimatter-particle-pair was able to survive long enough to be detected, for instance due to relativistic effects, as explained e.g. by the Hyperphysics page on muons: ...


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