Materia Incognita

Post by Tom Kitching, MSSL

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Occasionally people ask me what I work on, and at some point in the conversation the words “dark matter” inevitably arise. Normally one would say that dark matter “is named because it does not emit or absorb light, unlike ordinary matter that does…”. However in every day usage an object is “dark” because either i) it is absorbing light, or ii) there simply is no detectable light present (its dark at night because your eyes can’t detect the very low level of optical photons around).

So if everyday things are dark because they do absorb light, why does dark matter not absorb light?

Dark matter is a material that constitutes the majority of the mass in the Universe, it doesn’t interact with other particles with light but it does with gravity. Light in fact passes straight through it, as does the atoms that make up the Earth and you and all the stars and galaxies we can see.

Where did the phrase “dark matter” come from? Wikipedia tells us the familiar story told to generations of new physics students that:

Astrophysicists hypothesized dark matter due to discrepancies between the mass of large astronomical objects determined from their gravitational effects and the mass calculated from the “luminous matter” they contain: stars, gas, and dust. It was first postulated by Jan Oort in 1932 to account for the orbital velocities of stars in the Milky Way and by Fritz Zwicky in 1933 to account for evidence of “missing mass” in the orbital velocities of galaxies in clusters.

In fact Zwicky referred to the matter as dunkle Materie that has a  German-to-English translation of dark matter. It is interesting to note that dark matter did not enter the common scientific canon, or popular culture for another 50 years after Zwicky’s discovery. A search for the term “dark matter” in books produces the figure below, where it can be seen that it was not until the 1980’s that the dark was popularised. This was due in a large part to the work of Vera Rubin in the 1970’s on galaxy rotation curves, that lead to the publication of an influential paper in 1980.

Search on https://books.google.com/ngrams/ for "dark matter". The percentage of books that contain this term.

Search on https://books.google.com/ngrams/ for “dark matter”. The percentage of books that contain this term.

Astronomers are used to seeing matter that is emitting light, primarily stars, so it is only natural to label everything else (that is not actively emitting or absorbing light) as being “dark”. So, we do have an excuse.. But the problem is that the word “dark” whilst, arguably, is descriptive of our ignorance, is not physically an accurate description when “dark” is used in its everyday form. So, when trying to describe the physics of dark matter the word “dark” can cause confusion. In fact as a noun the Oxford English Dictionary has 5 definitions for the word dark:

  • 1 a. Absence of light; dark state or condition; darkness, esp. that of night,  b. The dark time; night; nightfall., c. A dark place: a place of darkness.,
  • 2. fig. (a leap in the dark)
  • 3. a. Dark colour or shade; spec. in Art. a part of a picture in shadow, as opposed to alight.,  b. fig. A dark spot, a blot.
  • 4. a. The condition of being hidden from view, obscure, or unknown; obscurity. in the dark: in concealment or secrecy. b. Obscurity of meaning
  • 5. in the dark: in a state of ignorance; without knowledge as regards some particular fact.
So on the face of it “dark matter” satisfies several of these definitions, except that  it doesn’t quite fit any of them. Dark matter is not an “absence of light”, since light can pass straight through it: dark matter can in fact be permeated with light. Dark matter is not a “leap in the dark” since it is not a wild or speculative conjecture (although some may argue on this point). Perhaps the most appropriate is definition 4, but in fact dark matter is not hidden from view, since with gravitational lensing we can directly observe its effect.  And with definition 5, the usage is not quite correct, which should be “we are in the dark, about this matter”.  Furthermore the name is somewhat depressing, one of the OED adjectival definitions is “Devoid of that which brightens or cheers; gloomy, cheerless, dismal, sad.” (and of course no one wants to turn to “dark side”).

Direct imaging of dark matter in the “bullet cluster”, blue is dark matter density and red is X-ray gas. From http://apod.nasa.gov/apod/ap060824.html . Composite Credit: X-ray: NASA/CXC/CfA/ M.Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al. Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.;

So perhaps we should look for alternative word to describe the matter with which the Universe is suffused, through which light can travel unhindered, and that acts as the cosmic scaffolding around which ordinary matter clusters. My humble proposals are either:
  • Transparent Matter : This has a nice descriptive word that actually has physical interpretation that is accurate. Clear matter would be along similar lines. If you are cosmologist would you prefer to be talking about transparent matter every day, or dark matter?
  • Materia Incognita : The unknown material. Maps used to be labelled with “terra incognita“, in a similar way we could be explicit about ignorance of its nature (caveat: my latin is possibly entirely incorrect). Maps also, according to legend, used to use the term “here be dragons”, so I am tempted to propose that dark matter should be called “Dragonite“, “Dragonium” or perhaps even “Smaugite“, but I won’t.
  • The Cosmic Scaffolding : This would explain an aspect of the role this matter plays in the growth of large-scale structure, but normal scaffolding can be seen, so perhaps “the transparent cosmic scaffolding”, but this is admittedly not as catchy as dark matter.

However the name dark matter is probably here to stay for the near future. But don’t worry because this will hopefully be a brief period as new direct detection experiments, and new cosmic sky surveys come online we will learn the true nature of this mysterious component of our Universe and give it its true name.

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