By Emily Levesque, University of Washington
Dark matter. These two words might conjure up fantastical ideas from science fiction, vague imaginings about the contents of the universe, or questions about how much we really know about the mysteries of space. It might sound esoteric but as far as we can tell, dark matter makes up about 85% of all the matter in our universe, and we see signs of it everywhere.
The Signs of Dark Matter
Since its discovery in the 1970s, we’ve learned a lot about what dark matter does. We see signs of dark matter in our study of the cosmic microwave background, and in the shape of our own galaxy.
But, as its name would suggest when it comes to seeing this ubiquitous part of our universe, we’re in the dark. Dark matter does not appear to interact with light in any way. It doesn’t emit light at any wavelength, so we don’t detect any faint hiss in radio waves or see any dim glow in visible light.
It also doesn’t absorb light, so we don’t see any telltale shadows or black clouds alerting us to its presence. As far as we can tell, dark matter doesn’t do anything to light at all and we’re still not sure why that is.
The Origins of the Phrase
The phrase dark matter was first used by French mathematician Henri Poincaré, who used matière obscure to describe dark hard-to-observe matter. Similarly, Swiss-American astronomer Fritz Zwicky used the term dunkle materie—dark matter again—to explain his observations of an enormous cluster of galaxies. The galaxies, he claimed, had far too little visible mass to explain their ability to stick together in a cluster. Some vast amount of unseen mass must be responsible.
The name stuck, but it’s a bit misleading given what we know now. Both Poincaré and Zwicky simply imagined dark matter as, well, dark: incredibly cold dim stars or clouds or gas, made of the same normal matter as everything else but simply very difficult to observe. Physicists now believe that dark matter is a distinct and unique type of matter that doesn’t interact with light at all.
A very dim star or a cloud of cold dust would emit at least a tiny amount of light, or block light from background objects. Even something like a cluster of black holes would have an observable signature by way of how it interacted with light—we’d see the gravity of those black holes bending the light that passed near them. What we call dark matter might be better imagined as invisible matter: it doesn’t interact with electromagnetic light at all.
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The Spiral Shape
We’ve still seen plenty of evidence for dark matter thanks to the effects of its mass. There are other telltale signatures of dark matter. Many spiral galaxies, even with their halos of dark matter, appear as flat spirals, but others look a little bent, like someone reached out and twisted the disk of the galaxy.
This shape comes from little satellite galaxies orbiting around the larger spirals and passing through their dark matter halos. The motion of those satellites through the larger galaxy’s dark matter can actually warp the shape of the spiral’s disk. We’ve observed examples of this in other galaxies and have even seen this telltale warping in the disk of our own Milky Way!
Gravitational Lensing
We also see the effects of dark matter thanks to something called gravitational lensing. All matter has mass, the mass has gravity, and gravity can bend spacetime. When spacetime is bent, the light that travels through it follows that bent path, making it look like it’s traveled through a curved lens: we call this gravitational lensing.
This lensing effect is especially dramatic when caused by extremely massive objects like huge black holes or enormous galaxies. Studying the bend of the lens can tell us how much mass is there, warping spacetime, and we know from studying gravitational lenses that some galaxies are much more massive than they appear, another sign of dark matter.
Cosmic Microwave Background
Even the cosmic microwave background supports the existence of dark matter. The anisotropies—tiny hot and cold patches that deviate from the model of a perfectly uniform early universe—are best explained by a model for our universe that includes the expansion of the universe and the presence of something known as cold dark matter. As with the word dark, the word cold here is a bit of a misnomer. It refers to the motion of dark matter in the early universe and suggests that the dark matter in this model couldn’t have moved very far.
In this model, dark matter helps explain how the first galaxies in the universe formed, and the theory agrees excellently with our observations of the universe. Jim Peebles, one of the astrophysicists who studied the cosmic microwave background, won the 2019 Nobel Prize in part for his work on the theory of cold dark matter.
Altogether, there are more than a dozen different observations that demonstrate the existence of dark matter. About 85% of all matter in the known universe is dark matter, and it’s everywhere we look, even if we don’t see it directly, dominating the mass of galaxies and driving how these galaxies and the properties of the universe have evolved. We now know that this mysterious invisible matter plays a crucial role in how the cosmos works.
Common Questions about the Evidence on the Existence of Dark Matter
The term dark matter was first used by the French mathematician Henri Poincaré. He used the term matière obscure to describe a dark matter that was barely visible. The term was similarly used by Fritz Zwicky, a Swiss-American astronomer, to describe his observations of a massive cluster of galaxies. Astronomers still use the term today, although with a different meaning.
All matter has mass, and mass has gravity. Gravity can bend space-time, and when space-time bends, the light that passes through it follows a curved path, and it seems to have passed through a curved lens called gravitational lensing.
About 85% of all known matter in the universe is dark matter. Dark matter is actually everywhere, even if it is not directly visible. It also shows us how the galaxies and the properties of the universe have evolved through time.
