At what point in science is does it become unreasonable to prop up hypothetical substances or entities? The title of this post is a tongue-in-cheek poke at a couple of well-entrenched, yet wholly unsubstantiated, such entities. This week Nature News reports optimistically on the continued absence of evidence for dark matter:
Physicists have again returned empty-handed from a search for the ‘dark matter’ that is thought to fill the cosmos. But the latest null result hasn’t dimmed their enthusiasm — or their plans for a new generation of detectors.
Since the 1970s, theorists have predicted the existence of massive particles that rarely, if ever, interact with normal matter. This dark matter is believed to be responsible for slowing the rotation of galaxies and makes up about 85% of matter in the Universe. Physicists have devised a host of experiments to find dark matter, but to date, nobody has been able to detect it directly (see Nature 448, 240–248 ; 2007).
The latest non-findings met with spontaneous applause at a conference on 22 February in Marina del Rey, California.
Both dark matter and the multiverse are hypothesized to exist on the grounds that they could explain particular phenomena. In the case of dark matter, that phenomena include the rotations and motions of galaxies and larger scale cosmological systems which do not agree with current theory.
In the case of the multiverse, the phenomena is related to the peculiar nature of quantum mechanical systems. But in what sense does the multiverse explanation offer a solution if it means an ad hoc postulation of an infinity of universes? On such a theory d’Espagnat writes:
It provides us with a consistent, logical framework of rules and concepts and shows that real physics can be inserted completely into this framework. Seen from a somewhat different standpoint, this theory also has the very unpleasant feature that, in a way, it runs counter to the principles of economy of assumptions (Occam’s razor), which is otherwise known to be so important in science. Indeed, it does so without any restraint, since it goes as far as to postulate infinities of completely unobservable worlds or branches thereof. An answer to this objection is that if the theory thus lavishly multiplies universes (or at least, branches of the Universe), it economizes on the fundamental principles, as we have shown.
The postulated ‘dark matter’ similarly offers the trade-off of multiplying substances in the Universe for the sake of economizing on fundamental principles. In this case, though, “economizing” takes the form of avoiding the need to rework existing fundamental principles. In other words, the dark matter postulate seeks to resolve the existing discrepancies between theory and observation by holding fixed the existing mathematical theory of physics while grossly varying the material contents of the Universe via addition of a new substance, one which both greatly supersedes in total mass the sum of all other known matter, and is also conveniently nearly impossible to detect; rather than holding fixed the material contents of the Universe in accordance with what has actually been observed while varying the existing mathematical theory of physics.
Varying the existing mathematical physical theory carries a certain repugnancy to the extent that such theory, once established and empirically supported, is ideally desired to remain fixed – invariant to time, context or scale, as tacitly implied by the common phrase “laws of physics”. Historically, though, physical theory has changed from time to time, in response to encountering new phenomena.
Now, the fact of such historical changes is clearly not, in itself, evidence that it is the physical theory that is in need of amending. But by the same token, neither is it evidence that the physical theory is inviolate and that it is, preferentially, the substances of the Universe that are in need of amending.
 Brumfiel,G. 2008. “Bright hopes pervade dark matter”. Nature 452:6-7. doi:10.1038/452006b
 d’Espagnat,B. 1999. Conceptual Foundations of Quantum Mechanics. Perseus Books