He clarified for me the point about the Bullet Cluster. What this shows is that the bulk of the gravity of the clusters separated out from the bulk of the normal matter (the gas and dust clouds – the stars actually have minimal mass percentage wise).

MOND cannot explain this – gravity separating from matter. Dark matter can – because the gravity is actually dark matter that can separate out from the gas and dust clouds, because the gas clouds collide and slow down, while the dark matter passes right through.

This sounds like pretty much a slam dunk for dark matter.

One last question for you: could the “weird thing going on” be vacuum fluctuations? I have to imagine that someone’s checked for this, but it (naively) seems to me that this all might be a gigantic example of the Casimir effect. So, I guess what I’m asking is: could dark matter just be virtual particles?

Dark matter is still more likely than MOND, GR and newtonian gravity are based on geometric properties, you would need something weird going on for geometric properties not to hold (like a field that permeates the entire universe similar to the ether). But at this stage it could be anything, we really have no idea.

I don’t think that’s a fair assessment of science. Scientists are saying, “We think this is how it works,” and then they test it and it’s correct up to a certain point. So they look at it and say, “Well, it’s not right on this really large scale, what are the possible reasons?” And they come up with more tests. That’s what we’re seeing going on today. Some scientists are testing the dark matter theory, and others are testing MOND. (And there may even be others testing something else.) In the meantime, people who need to get work done can use whichever model gets them the best (i.e. most correct) results for their work, until we can figure out which is correct. And that work may give further information telling which is more correct, too.

This is completely different from theist who often say, “Well, I can’t explain this, and I don’t feel like trying, so God did it!” Since science is always provisional, scientists aren’t really saying, “It works this way.” They’re saying, “As far as we can tell, it works this way, and here’s how accurate our model is, and here’s how we know that.” Whereas, theists often say, “It works this way because someone claimed it does.”

If you don’t think dark matter or MOND are correct, you can go test them yourself. If you don’t think a particular religion is correct, there’s often no way to test that.

wb4, I’ve also wondered if the Pioneer anomaly could be explained by this. cottreau, do you have a source for the claim that the probes are off by the same relative amount. (Not calling you out, just wondering.) I sometimes hesitate to bring up the Pioneer issue, because it seems to bring the crazies out of the woodwork. I’ve dealt with folks who can only be called modern physics deniers in the past, and they LOVE the Pioneer anomaly.

As for dark matter as a whole, I’ve always found it fishy. Most of my knowledge is secondhand, though. I just always got the impression that all the evidence for it that they have is just as well explained by gravity being off at large distances and/or times. For instance, my ears always perk up when Pamela Gay goes over the evidence for dark matter on AstronomyCast … and then I just think to myself “couldn’t gravity being wrong explain all that just as well.” And then Frasier mentions that “we have a map of dark matter,” and I think “well, that’s either a map of dark matter, or a plot of error vs position.”