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As I understand it, matter that is too distant for its light to reach us would not be observable, and the CMB is the limit of what is observable from our location within the universe. Could what is known as "dark matter" simply be matter that is beyond this horizon? Is there a specific reason that dark matter must be located within our observable horizon?

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We can see regular matter in some galaxies whose rotation curves indicate that they have dark matter in them. Thus it cannot be the case that dark matter is simply too far away to see.

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  • $\begingroup$ There's also gravitational lensing by clusters. Whatever dark matter is, it must be in those clusters to cause lensing. And those clusters are definitely within our horizon. $\endgroup$ – hdhondt Mar 11 at 8:51

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