It is, indeed, more than a bit threatening.

(Still, if Milton was able to describe it, scientists ought to be able to map it....)

Not quite. Astrophysicists use pictures (lots and lots of them) to look for movement that can't be explained by what they see - then they know there is some gravitational force acting on a visible object that has already been charted.

To stretch your analogy, it would be like finding munched up bones, torn up flesh, and perhaps some vomit and other detritus, and then drawing the monster on the map. After all, that's how giant squid were 'discovered', isn't it?

Gravitational anomalies mean that something is present, but "dark matter" and "dark energy" are probably placeholder terms at best until the real culprit is discovered.

More and more anomalies showing up usually signals that the current model is actually wrong and something else needs to replace it entirely. It's not just a matter of a few patches.

I suspect the real explanation will change physics in much the same way that Copernicus and Kepler overturned Ptolemy and Einstein overturned Netwon.

Are there Heffelumps out there?

Someday "dark matter" will take its rightful place alongside phlogisten, epicycles and "the aether".

Jim:

Or observing the fall of a meteorite, and concluding that it must have been ejected from an invisible volcano.

jim:

No, it's how my fraternity's Super Bowl Party was found. Illustrates the point nicely though.

Remember, the neutrino (the 'baby' neutron) was just a construct for a long time. They have been observed (but it is a cast-iron b**** to do so).

rat:

Hoax.

The neutrino a hoax? Hardly. Go back to Physics 401. Do not pass GO, do not collect $200.

Neutrinos were first directly observed in 1956, and have been detected in various ways since.

Ratbert, Neutrinos cannot be directly observed. Thus the very elaborate tests to find them. While some Neutrinos may have been detected, by our current understanding, the ratio of Neutrinos that we belive exist to Netrinos that have been detected is off by a factor of what, a billion+ to one. Something is way off with either the test or the theory. Either way, OJ's statement is creditable.

What I don't understand, looking at their map, is: If the majority of mass is "dark matter", why doesn't it clump as much or more than the ordinary matter?

Their mapping relys heavily on the assumption of uniformity, attributing anomalous galaxy shapes to gravitational lensing by "dark matter". But in a sample of half a million galaxies going back 6.5 billion years, there are bound to be many anomalous galaxies.

This result is not as clear-cut as the lede implies - there are "small" problems lurking in the dark. "Concentrations of ordinary matter almost always overlap with concentrations of dark matter - but not absolutely always. Conversely, the researchers saw that dark matter concentrations sometimes seemed to have no corresponding ordinary matter. 'It's not forbidden, but you get a little uncomfortable because you would think the two should go together,' said Dr Linder."

Not to worry though: "Dr Massey ... told BBC News, 'the discrepancies are not yet at a level of significance where I am definitively convinced they are something other than noise or isolated defects in our analysis.'" If the discrepancies are noise or defects in analysis, how do we know the whole thing is not due to noise or defective analysis, Dr. Massey?

This strikes me as an exercise in circular logic: Begin by assuming "dark matter" and that any variations are solely due to it, observe variations (quelle surprise), and cite the result as "proof" that "dark matter" really does exist.

Robert:

Of course they are difficult to detect: they are virtually massless, and carry no charge.

The first experiment to 'observe' neutrinos involved an inverse detection, by monitoring the flux of anti-neutrinos as neutrons decayed in a nuclear reactor (via bombardment of the hydrogen nucleus). Subsequent tests (that I am aware of) include installing huge water tanks deep underground (in rock, to shield them from cosmic rays) and then looking for that one-in-a-billion interaction.

But remember, there are billions of neutrinos passing through the earth every second. So, the detection of an interaction is not as far-fetched as it sounds. It has been years since I read about the tank tests, but I believe they were recording approximately a hit a month or so, which is a reasonable enough result.

Ratbert, I know, which is why I commented on your "directly observed" comment. I also know that the numbers of Neutrinos detected is so far below what is expected that some have speculated that the Sun has turned off.....

rat:

Your quotes concede the argument. But that's at least honest