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Comment Re:String theory is just that: a theory (Score 3, Informative) 161

The observations are not wrong. Measuring the velocity dispersion of stars in a globular clusters is not a much harder, or more conceptually difficult, than measuring the colour of a fluorescent light. Determining a galactic rotation curve is a bit more complex, but not much so. These observations have been done tens of thousands of times using many different techniques, sometimes by groups of astrophysicists who hate each other and would like nothing more than to discredit the person who got to speak instead of them at last January's AAS meeting. The observational evidence for dark matter is overwhelming. The modeling, on the other hand, has more assumptions built in. The key assumption is that gravity, in the weak limit, follows Newton's law of gravity, and there is a 400 years of evidence to support that.

Comment Re:Great news everyone (Score 4, Informative) 161

Many people have spent a lot of time looking for ways to explain single like galaxy rotation curves, stellar velocities in globular clusters and elliptical galaxies, the structure of galaxy clustering and what-not without success. The simplest explanation has always turned out to be that there is some sort of extra matter that we cannot see. Dark matter requires the smallest number of assumptions out of all explanations that people have proposed so far. By Ocham's Razor it is probably the right solution. And by Grabthar's Hammer you shall be avenged.

Comment Re:Or it could be, you know, measurement errors (Score 3, Informative) 274

> Frankly, "dark matter" is like "magnetic monopoles". It works in mathematical models, but hasn't shown up in experiments and is not a *necessary* to explain how things work. Simpler models are powerful and elegant enough to cover the existing structure.

I am breaking my usual rule of not responding to anonymous cowards, but the quoted statement is wrong at several levels and I am feeling masochistic this morning.

The idea of dark matter does not come from mathematical models, it comes from observations. The standard model of particle physics does not predict dark matter. Dark matter was detected in experiments (or observations if one wants to be pedantic). There is no theoretical basis for dark matter, but there is a large body of evidence, from many different types of observations, supporting the idea that dark matter is a real part of the Universe. At present there are no theoretical alternatives to dark matter than can reproduce what we observe in the sky. Unless the past 80 years of observations are wrong then dark matter is necessary to explain what we see. There are no simpler models. Many have been tried, including small- and large-scale changes to gravity, and none have been able to reproduce what we actually observe.

Dark matter is not simply a measurement error. There are too many independent observations that all point to the existence of dark matter. Not only that, they all point to the same amount of dark matter and require that similar properties for dark matter. Measurement errors do not always work in the same direction across vastly different types of measurements. Bib Bang nucleosynthesis and the COBE/WMAP/Planck observations are completely different from galactic rotation curve and cluster velocity dispersion measurements, and yet they all predict consistent amounts of missing mass. Stray planets and low density clouds of cold gas are not enough to close the gap. Even if they did work for galactic rotation curves they would not be able to explain the results of the cosmic background radiation observations.

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