Staying with astrophysics (qv), let’s think about dark matter and light years. As the Wikipedia entry states, “Dark matter is a hypothetical kind of matter that cannot be seen with telescopes but would account for most of the matter in the universe.” The subtitle of an article in Science in 1984 was, “It fills the universe, it is utterly invisible, and it may not even exist; meanwhile, the hypotheses are getting more exotic.” When things get extremely large or extremely small, the human mind sort of reels a bit. I suspect those are not hypotheses, because hypotheses are testable; these are speculations, if not wild guesses. The last section of Wikipedia’s entry is about dark matter in fiction when, “it is usually attributed extraordinary physical or magical properties. Such descriptions are often inconsistent with the properties of dark matter proposed in physics and cosmology.” But as, so far, we don’t really know what those properties are, it might as well be fiction.

Dark matter in medicine implies ignorance on a grand scale. It works in some circumstances better than others, and is a much better metaphor than black hole (qv). It is a common metaphor in genetics and the developmental sciences, applied to apparently non-coding nucleic acids, although that is not really analogous: we know the DNA or RNA is there, we just don’t know what it does. Similarly, there are “dark neurons” in the brain which are thought to fire off to very specific stimuli, but for which there cannot yet be direct evidence. Dark matter applies less well to the unknowns of social interaction, because here we are far too far away from any physical understanding. And trying to apply it to unexplained triggers for cancer as “dietary dark matter” is ill-considered.

There is an intriguing abstract in PubMed attributing the development of homochirality (that many biological molecules have a handedness) to cold dark matter and dark energy. Proof would be helpful: the failure of science to explain homochirality is taken by some to be evidence for the supreme creator.

A light year is a measure of distance. Policy cannot be light years behind technology, nor can someone be light years ahead of their time, and—unless currently on a galaxy far, far away—nor can a proper measurement of surgical outcome be light years away. Informally, we may say something is a long way off, otherwise a different metaphor is needed for a very long time. If not a metaphor, why not take a word appropriated by geology—aeons?

Another geological term is epicentre, which is the point on the Earth’s surface vertically above the focus of an earthquake. As such, it shouldn’t be used merely to replace the word focus, or centre. But epicentre has long been used to mean somewhere, or even something, of central importance: the epicentre of British medical politics (which could be the BMA or Tavistock Square), the epicentre of the Ebola epidemic (which may be different from its origin), or the epicentre of East-West confrontation.

While looking for metaphorical light years, I found an article titled, “Scaled-up darkness. Could a single dark matter particle be light-years wide?” You wouldn’t want to bump into one of those after a hard night on call.

Competing interests: I have read and understood BMJ policy on declaration of interests and declare that my only competing interest is my co-authorship of a book about medical English.

Neville Goodman is a retired consultant anaesthetist and a writer, and co-author of a book on medical English.