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Comment by JohnBooty | Hacker News Reader

EDIT: My understanding is incorrect. See the replies to the post for a correction.

No. They're saying that 50% of the "dark matter" isn't dark, it's just regular matter that we weren't able to see.

The density of this stuff (filaments of gas stretching between galaxies) is so low that it is really, really, really hard to detect.

"Dark matter" theories posit that the missing matter isn't just regular matter that's hard to see; they posit that this dark matter is a fundamentally different... something.

I don't believe this announcement addresses some of the other oddities (possibly) explained by dark matter, such as the rotation of galaxies: https://en.wikipedia.org/wiki/Galaxy_rotation_curve

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> They're saying that 50% of the "dark matter" isn't dark, it's just regular matter that we weren't able to see.

No, the dark matter proportion hasn't changed. There was missing "regular" matter that has now been accounted for.

Thanks for the correction. I updated my post with a note pointing out that it's wrong.

No, this has nothing to do with dark matter. Dark matter has to be in galaxies because the reason we think it exists at all is because galaxies spin 'too fast', so there is more mass in them and therefore more gravity than can be accounted for. This article is talking about gas between galaxies.

Isn't it more probable that the formulars to calculate the spin are wrong?

That's a theory called Modified Newton Dynamics (MOND).

https://en.m.wikipedia.org/wiki/Modified_Newtonian_dynamics

This theory (when appropriately hacked) describes rotation curves of galaxies extremely well. Unfortunately it doesn't work at larger scales. There are many nice videos on YouTube by Sean Carroll on MOND, which explain it's limits, albeit for the fairly educated layperson.

Its possible but there is a lot of indirect evidence for dark matter, it fits observation well even if we cant prove it. When gravity calculations give an answer that conflicts with observation, sometimes the equation for gravity ends up being wrong (orbit of mercury) and sometimes you find missing mass (discovery of neptune). I don't know what more probable means in this context, the experts are at least as smart as you and me, they favor some kind of missing mass.

But we know they "spin too fast", by measuring the amount of observable matter and then measuring the "rotation curve" of the galaxy. However, the basic point that this is not half the dark matter, is correct, (based on our current understanding). Even if we can't see ordinary matter, we should be able to see its effect in some way other than gravitational interaction. Dark matter is only seen by its gravitational interaction. It doesn't interact electromagnetically.

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