On the possible existence of “dark matter” you may have seen headlines about a recent “proof” of dark matter based on data from a collision of galaxies. The title of the paper, to be published in the prestigious Astrophysical Journal, is “A direct empirical proof of the existence of dark matter” (Clowe et al., 2006). Upon reading this title, I became immediately suspicious because, as philosopher of science James Hall states, “Our hypotheses may get support or they may go down in flames, but they never, ever get proved”. The paper features some impressive technical discussion, but contains no discussion of some critical caveats. In particular, the argument assumes that normal matter is fully accounted for by the inventory of visible stars and hot plasmas. However, it has been reported that non-visible interstellar gas, lower-energy plasmas and brown dwarfs, in combination, likely exceed luminous stars in the local mass budget of our own galaxy (e.g., Fuchs, 2006 ref;
http://adsabs.harvard.edu/abs/1998A&A...339..405F).
