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Let me start out by saying that while I read a lot about astrophysics on Wikipedia and in the news and find it very interesting, I never really got into the calculations because I never took a course in it or read a textbook. I have a PhD in math, so if there is any mathematics that can enhance an answer to this question please don't hesitate to include it.

Something that has always bothered me was dark matter. It seems like a "cheap fix" to try make observations coincide with known theories of gravity, and it seems to be widely accepted that this is a better idea than revising the theories of gravity to match observations. Let's accept that for now.

It is assumed that if dark matter exists, it is exotic and literally does not interact with electromagnetic radiation at all, or at least very minimally. Why does this have to be the case, when instead the "dark matter" could just be ordinary matter that's floating around in the universe, but is not a star and is not near a star or any source of light, so no electromagnetic radiation is emanating from it, and hence we can't see it with telescopes? That's my first question.

The second question is...what about black holes? Could they account for the matter that is missing?

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  • $\begingroup$ "...but is not a star and is not near a star or any source of light, so no electromagnetic radiation is emanating from it..." You don't have to be a star to emanate radiation. In fact, any normal object at any finite temperature at least emits thermal radiation. $\endgroup$
    – hft
    Commented Jul 27, 2022 at 23:28
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    $\begingroup$ @hft I've read about Hawking radiation, and it is my understanding that it has never been detected and in general is too minuscule to detect. $\endgroup$
    – Matt Samuel
    Commented Jul 27, 2022 at 23:30
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    $\begingroup$ Sure, but to even emit any electromagnetic radiation, however miniscule, you still have to couple to the electromagnetic field a little. I think the idea behind "dark matter" is that the word "dark" means it does not couple to the electromagnetic field at all. So, it can't be made up or protons or electrons or positrons or, well, any type of elementary particle that couples to light. So you go thru the Particle Data Booklet (pdg.lbl.gov) and look for what doesn't couple to light and what also might be heavy enough to account for whatever gravitational effect dark matter has... $\endgroup$
    – hft
    Commented Jul 27, 2022 at 23:43
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    $\begingroup$ ...and you don't find anything known to current physics that fits the bill (or maybe you do, I don't know). So, you throw up your hands and say, "dark matter." $\endgroup$
    – hft
    Commented Jul 27, 2022 at 23:44
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    $\begingroup$ The problem is that you need a lot of invisible mass to account for galaxy rotation curves. Plenty of people aren't fond of dark matter, so a lot of work has gone into finding alternatives, but it's really hard to scrape together enough mass. $\endgroup$
    – PM 2Ring
    Commented Jul 28, 2022 at 0:00

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