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Comment by netcraft | Hacker News Reader

is it possible that dark matter is regular matter, just so diffuse that we don't notice/see it? could it be hydrogen atoms just floating in empty space, at such low densities we would never notice but in the grand scheme of things add up? Edit: I dont mean to propose a potential solution here, it was more a question of why it must be unknown matter, why can't it be normal matter.

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No, it is not possible. Cosmological models are actually tightly constrained by data. Even in the late 80s, when we did not have such good data, I remember being amazed at the fact that primordial nucleosynthesis calculations together with observational data were enough to conclude that the number of light "normal" neutrino species was 3, while there were still speculations in Particle Physics that we could have more than 3 generations of particles. The data accumulated over the last 30 years has constrained things even more.

If some other explanation than invoking some new type of matter that can not be seen was possible to explain the observational data, don't you think that Physicists would use that instead of coming up with postulating that the Universe is dominated by "dark matter" and "dark energy" which we have not seen yet?

Looking back at what I wrote I can understand your reaction - im wasnt trying to propose a new solution that someone hadn't considered. It was more a question of why couldn't it be that, which you answered quite well, thank you.

Neutral hydrogen in space is actually quite readily observed via radio telescopes as the famous spin-flip 21cm line. This signal has been very useful in mapping the distribution of neutral hydrogen within our own galaxy and those of our galactic neighbors.

In addition to this, there exist many arguments in favor of classifying normal matter and dark matter separately.

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