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Skip to 0 minutes and 13 seconds Dark matter was first identified by Fritz Zwicky in 1933. At the time, Zwicky was studying the Coma cluster, which is a cluster of something like 1,000 galaxies. And he was measuring the individual velocity of each of the galaxies. We have seen that galaxies are receding from one another because of expansion, but when they are close together, like in a cluster, they are submitted to gravitational attraction, and they could have particular motion. And so it was by identifying the individual velocities, due to the gravitational attraction, that Zwicky wanted to measure the mass of the Coma cluster.

Skip to 0 minutes and 58 seconds And to his surprise, he identified that the mass was something like 400 times larger than what was measured from the luminosity of the individual galaxies. And so that meant that in that cluster, there was much more mass than the luminous mass of the galaxies, and this was the birth of what we know now as the dark matter.

Skip to 1 minute and 27 seconds It was later found out that this is not specific to the Coma cluster. It is not even specific to clusters. There is dark matter– that means a non-luminous form of matter inside galaxies, even. The way to identify the presence of this dark matter is to look at what we call the rotation curve of stars around galaxies. Galaxies have a rotational motion, so stars in the galaxy are rotating, and this is due to the gravitational attraction. Now, one can identify this velocity. And one imagines that, as one gets away from the centre of the luminous centre of a galaxy, there is less luminous mass, and so the rotation velocity should be slower. Now, this is not what happens.

Skip to 2 minutes and 22 seconds And one realises that in all galaxies, as one gets to the outskirts of a visible galaxy, the rotation velocity is still very large, which means that there is still gravitational attraction. There is still some form of mass, non-luminous mass, because we are at the boundary of a luminous galaxy. And so it is this observation in basically all galaxies that led to the conclusion that there is a significant amount of dark matter inside galaxies.

Skip to 3 minutes and 4 seconds So let us take as an illustration the example of our own galaxy, the Milky Way. You have here a picture of our own galaxy, which consists of some 10 to 11 stars, so approximately 100 billion stars, which are made of course, of luminous matter that emit light. And these stars are mostly structured in a flat disc and a central bulb. You also have globular clusters around that central structure, but that’s what is the visible galaxy. But on top of that, there is, as you see, a spherical halo of dark matter, which is responsible for the gravitational attraction on distant stars, responsible for the rotation curve that we have seen earlier.

Skip to 4 minutes and 1 second And so you see that a typical galaxy has this spherical halo of dark matter surrounding the visible part of a galaxy.

Skip to 4 minutes and 14 seconds Another way of identifying the presence of dark matter is to use the curvature of light rays by the presence of matter. Whether luminous or dark, it will curve the light ray in the vicinity. We have already seen this beautiful picture of the Hubble Telescope, which shows arcs, which are just deformations of the images of distant galaxies by the presence of matter. So let us see in more detail what is the principle of the formation of these arcs of light.

Skip to 4 minutes and 50 seconds Just consider a distant galaxy, which is observed by us on Earth.

Skip to 5 minutes and 4 seconds And imagine that there is a concentration of dark energy in between.

Skip to 5 minutes and 11 seconds Then the light rays limited by the galaxy will be slightly deformed, and so you see that the concentration of dark matter is acting like a lens. This is why one calls this phenomenon gravitational lensing. And so at the end, because of this distortion, we do not observe this spherical galaxy, but we observe a distorted image of the galaxy and of a shape of an arc. And precisely by looking at the deformation, we can identify the concentration of dark matter. And so indeed, if we have several galaxies, we’ll be able to identify exactly what is the form of this concentration of dark matter. Now, that has been used to make maps of dark matter.

Skip to 6 minutes and 13 seconds Recently it has been used by the Planck Collaboration, this time not using distant galaxies, but using the photons of the CMB that you see in this picture here. The photons of the CMB encounter dark matter when they propagate towards us, and so their trajectory is curved. And by studying the curvature of these trajectories, one is able to reconstruct maps of dark matter all over the sky. And this is what was produced by the Planck Collaboration using, again, this gravitational lensing effect.

Skip to 6 minutes and 52 seconds A spectacular illustration of the use of gravitational lens in order to detect dark matter is provided by the famous bullet cluster picture. In this picture, one sees the collision of two clusters of galaxies. In this collision, one could identify the dark matter part of the two clusters, and that is represented in blue in this picture. And this distribution of dark matter was identified through the gravitational lens effect. By looking at the X-ray emission, one could also identify– that’s represented in pink– the distribution of luminous matter. And you see that in the collision of the two clusters, dark matter and luminous matter are behaving a very different way.

Skip to 7 minutes and 45 seconds The two spherical halos of dark matter are sort of passing through the collision basically unperturbed, whereas the visible matter, the pinkish matter, is passing each other and is very much disturbed by the collision. And so one infers from this type of picture that dark matter is rather weakly interacting. So basically, when there is a collision between two clusters of galaxies, basically it does not interact neither with the other dark matter nor with the luminous matter.

Dark matter

Let me tell the saga of dark matter, this non-luminous form of matter, from its identification in the 1930s by Fred Zwicky in the Coma cluster to the realization that this is the most common form of matter in the whole Universe. (8:32)

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