The large-scale distribution of dark matter in our universe mapped by Hubble's Cosmic Evolution Survey. Photo courtesy of NASA, ESA, and R. Massey (California Institute of Technology)
Within the confines of the ordinary, vision is the most reliable tool we have. But some of the most extraordinary parts of nature, those that lie at the frontiers of science, can't be seen at all. Dark matter, for example, the invisible, mysterious material that makes up 22 percent of the stuff in the universe, is one of the great scientific unknowns, a substance nearly six times as abundant as ordinary matter but made up of fundamental particles we haven't yet identified. And dark matter doesn't emit light, it doesn't reflect light, and it doesn't absorb light. It's not dark, as the name suggests—dark matter is completely, inherently unseeable.
While we are unable to see dark matter itself, we are able to create maps of it, pinpointing
its location by observing the effects of its mass on light from distant galaxies. Einstein's general theory of relativity predicts that a massive object will curve the fabric of space, and light will follow this deformed path. So we can look at how light from galaxies has been bent and infer the quantity and location of the matter that did the bending. An international team of astronomers recently used this method to create the first three-dimensional map of the large-scale structure of dark matter, and they released this blue and black image earlier this year.
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In one sense, the image is categorically sublime. Scientists have taken something that cannot be seen, and they've let us see it. They've not only increased our knowledge of the large-scale structure of dark matter, they've also taken something inherently invisible and given it an accessible beauty. While we used to struggle to conceive of dark matter, using phrases like "pervasive stuff" and "ineffable material" to force it into the confines of language, we can now imagine it as a grand network with dense hubs and small offshoot islands.
But when we start to intuit dark matter rather than discuss it rigorously, we are in danger of imagining it as too much like ordinary matter. In fact, dark matter's invisibility tells us that it has another remarkable attribute: Unlike normal matter, dark matter is not made up of charged particles. Maxwell's equations, the four expressions at the heart of electromagnetism, tell us that moving charges can create electromagnetic radiation; they tell us that charge produces light. So because dark matter does not interact in any way with electromagnetic radiation, we can conclude that it is uncharged. This, in turn, implies that dark matter is nearly collisionless—individual particles don't bounce off of one another or off of ordinary matter; almost all of them just pass through as if nothing were there. We are constantly being irradiated by the billions of dark matter particles that pass through our bodies every second, but these particles will never touch our flesh or our organs. Were we able to see dark matter, we would be justified in assuming we could interact with it. So when we look at an image and perceive dark matter as a seeable substance, we implicitly imbue it with a property it doesn't have.
As people, we take in pictures with ease—absorbing visual information is, after all, our evolutionary specialty—but in any scientific case in which we see the unseeable, we sacrifice a little bit of understanding. To crib from Tolstoy, "All seeable things are alike; each unseeable thing is unseeable in its own way." Visible objects are all composed of atoms that emit light within a certain frequency range; they're all macroscopic; and they all exist in our three spatial dimensions. When scientists discover something invisible, the cause of its unseeability will inevitably be tied with what makes it exciting.