How do we know dark matter exists if it can’t be detected?

We know that dark matter exists because, although it does not interact with light — hence its name — it does interact with normal matter and has a gravitational effect on it. We call normal matter, or luminous matter, everything that we understand theoretically and that we can observe: planets, stars, even interstellar gas, and also ourselves, are made of it. We know this matter very well, we know that it is made of particles that we explain with the standard model of particle physics.

One of the effects of this gravitational interaction is that dark matter can change the trajectory of objects made of normal matter. We have different evidence for this and all this evidence tells us that dark matter represents about 85% of the matter in the universe. We have this evidence at different scales. For example, at galactic scales; also at the scale of galaxy clusters, which are much larger structures because clusters are made up of tens or hundreds of galaxies; and at cosmological scales, which are even larger still.

The first important evidence dates back to 1977, when the American astronomer Vera Cooper Rubin measured the speed at which stars rotated in a group of spiral galaxies. Applying the laws of gravity, one would expect the speed of stars further and further away from the center to decrease, since most of the normal matter is in the center of the galaxy. But what Rubin saw was that, very far from that center, the speed remained constant. The only explanation for this is that there had to be other matter that we did not see, and with a mass nine times greater than that of the matter we did see. This was the first evidence that irrefutably proved the existence of dark matter. And subsequent tests have yielded the same results. This is an example of galactic evidence, the smallest scale.

The evidence in galaxy clusters focuses on the measurement of matter through the gravitational lens effect. This effect is based on the fact that, according to Einstein’s theory of relativity, when photons — the particles that make up light — pass close to a massive object, their path bends. So, if we have a light source that we look at with a telescope and, between the source and the telescope, there is a massive object, you can estimate the mass of that object by measuring how much the light has bent. This is how matter is calculated in galaxy clusters. And what has been seen with this type of measurement is that only 20% of the matter is luminous; the rest does not emit light, so it is thought that it has to be dark matter.

There is also evidence on even larger scales, obtained from the cosmic microwave background. This radiation originated when the universe was 400,000 years old — it is now 13.8 billion years old — and we can measure it today. By studying it, we obtain information about the moment in the universe when structures began to form, just when photons began to travel freely and when the cosmic microwave background radiation originated. When this radiation is measured, its characteristics are only properly explained if we accept that there is more than 80% dark matter in the universe.

That is, all of these pieces of evidence at different scales and with different methods predict dark matter, and they predict the same amount of it. That's why we're so sure that dark matter exists, even though we don't know what it is yet.

Marina Cermeño Gavilán is a PhD in theoretical physics and researches dark matter at the Institute of Theoretical Physics in Madrid (UAM-CSIC). This article is part of an initiative sponsored by the Dr. Antoni Esteve Foundation and the L’Oréal-Unesco ‘For Women in Science’ program to answer readers’ questions about science and technology. This question was sent in by Mariana Pérez Pásaro.

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