Is it certain that dark matter exists? | Science

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Image of gravitational lensing produced by a cluster of galaxies, taken by NASA’s Hubble Space Telescope.NASA, ESA, K. Sharon (Tel Aviv University) and E. Ofek (Caltech)

There is evidence of the existence of dark matter for quite some time. The first time we had an idea that there was something else we didn’t see was in the twenties and thirties of the last century. But the clearest proof came with the studies of the American astrophysicist Vera Rubin on the movements of spiral galaxies, which are the basis for us to think that there is dark matter. To understand what Vera Rubin’s work means, you should know that we know the relationship between the mass contained in galaxies and the light they produce. That means that we can calculate how much mass a galaxy contains thanks to the light that reaches us from it. At the same time, we can also estimate the mass of a galaxy by studying its speed of rotation. What Vera Rubin observed is that the data she obtained thanks to that relationship did not add up. Much more mass than the visible mass was needed to account for the speed at which the galaxies rotated. This is considered the most classical evidence for the existence of dark matter, but there are many more.

The next most important perhaps is gravitational lensing. Thanks to the theory of general relativity, we know that a large mass causes light passing near it to bend. If there is an intense source of light, such as a quasar, at a certain distance and there is something with mass in the middle, the light that comes from the quasar reaches us with a certain curvature. In this case, the curvature that reaches us does not correspond to the amount of visible mass that we observe, but to a greater mass. But the amount of dark matter that we estimate to exist thanks to gravitational lensing is consistent with Rubin’s measurements of the spins of galaxies.

Another sign of dark matter comes from the distribution of the universe’s microwave background radiation, which we can also measure. What happens with this radiation is that it has a bit of an odd shape, it’s not symmetrical, but it has an anisotropy that doesn’t correspond to what we would expect if all matter behaved like ordinary matter. If all the matter in the universe were like the one that forms us and since the Big Bang everything had worked as we know ordinary matter behaves, the microwave background radiation would not be as it is. But if we take into account dark matter within that model, then it is consistent with what we see.

The observations we get from the universe are not possible with just the matter we know

That is to say, the observations that we obtain of the universe are not possible only with the matter that we know. So we deduce that there must be something else, something we have called dark matter. Although its name should be transparent matter because, at least for now, we cannot see it. We know it is there and we detect how it behaves, but we cannot see or describe it.

The reality is that we have no idea what it is. It probably has mass because what we detect seems to be caused by the presence of mass. Almost all the evidence is gravitational. We know then that it is affected by gravity and that it neither emits nor absorbs light. And, so far, we haven’t seen dark matter interacting with any kind of normal matter. We don’t know if it’s made of particles or if what we think is dark matter is gravity behaving in ways we don’t know about.

Just last year, an article was published that proposed defining dark matter as a modification of gravity. When this article came out proposing that there isn’t really dark matter, but that there are parts of the force of gravity that we don’t fully understand, there was a lot of excitement at first, but then responses were published pointing out things that the authors of the article didn’t take into account. So for now we still think there is dark matter.

The most natural thing from our point of view is to think that dark matter, like normal matter, is made up of particles

The most natural thing from our point of view is to think that dark matter, like normal matter, is made up of particles. This is how we understand the universe: made up of particles that interact with each other. If the dark matter were part of a dark sector that exists parallel to our own, there is no reason to think that it could not form structures like the normal matter that we see. But for now we have no way of checking if that is the case or not because dark matter, if it interacts with us, does so very weakly. What is in progress are many experiments of various kinds to try to see it: direct, indirect observation and its study in particle collisions. Precisely the latter is what I dedicate myself to, to look for a dark sector in collisions that we cause in the Large Hadron Collider (LHC) and study with the ATLAS particle detector. So far we haven’t had any positive results, but the space to search is still limitless.

Rebecca Gonzalez Suarez She is a doctor in high-energy physics and a researcher at Uppsala University in Sweden.

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we answer is a weekly scientific consultancy, sponsored by the Dr. Antoni Esteve Foundation and the L’Oréal-Unesco ‘For Women in Science’ program, which answers readers’ questions about science and technology. They are scientists and technologists, members of AMIT (Association of Women Researchers and Technologists), who answer these questions. Send your questions to us at [email protected] or via Twitter #nosotrasrespondemos.

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