Static and unchanging Universe vision was supported by the philosophical and scientific world for over 23 centuries. It's in 1929 that it collapses with the discovery of the expansion of the universe by Edwin Hubble (1889-1953). Today we know that physical objects (stars, galaxies and gas) that compose the universe are not only moving, but would be merely the visible part of an "iceberg", where most, 96% of mass and energy, would be hidden. Ordinary matter, dark matter, dark energy are the constituents of the Universe.
In 1933, the Swiss astrophysicist Fritz Zwicky, studied a small group of galaxies in the Coma cluster and found that the dynamic mass of the cluster is much higher than its visible light mass. At that time, the discovery is not considered revolutionary, the fact that it lacks a bit of matter in a galaxy cluster is not critical. But in 1978, an American astronomer Vera Rubin, in addition to his four children, focuses to the study of galaxies. She notes, on a smaller scale that of galaxies, than the stars in the outskirts of galaxies seem to rotate too fast. The rotation curve of some spiral galaxies was flat. The velocity of the stars does not was decreasing depending on their distance from the galactic center. However all the movements are related to the gravitational force, in the solar system further away from the sun, more gravitational force weakens and distant planets rotate slower than the planets close the Sun. It is the same for stars in galaxies where for galaxies in clusters of galaxies and in general to all bodies that revolve around an intense gravitational field.
For most of the scientists, there is no question of challenge the laws of the gravity, they consider star clusters, galaxies and galaxy clusters as gravitational systems in dynamic equilibrium.
When the stars have had time to do several times around a galaxy (crossing time), we consider that the galaxy has reached a state of gravitational equilibrium. Else the stars would have disappeared from the gravitational space of the galaxy.
The conclusion is that it lacks mass in the observable universe to ensure its stability. Various hypotheses have been put forward to explain the observed anomaly. Scientists have tried to add large masses in the form of molecular gas, dead stars, brown dwarfs, neutron stars, black holes, but it was not enough. Calculations show that the dynamic mass derived from the gravitational influence is at least 5 times larger than the visible mass in our telescopes.
In other words, Newton's theory is checked if there is a huge extra mass in galaxies. The huge missing mass is a kind of non-baryonic (protons, neutrons). Dark matter, until then unknown, became fashionable in the scientific world. However in 1983, the Israeli physicist Mordechai Milgrom challenges the concept of dark matter. He proposes a small modification of Newton's theory to solve the problem of too rapid rotation of the stars and galaxies. Mordechai Milgrom calls his theory "MOND".
Image: rotation curve of spiral galaxies. Blue (A), the rotation curve calculated using the equations of Newton, red (B), the curve observed depending on the distance of the stars relative to the center of the galaxy. Credit image: www.astronoo.com
MOND theory (MOdified Newtonian Dynamics), challenges the concept of dark matter and offers an explanation of the problem of flat rotation curve of spiral galaxies. MOND is based on a modification of Newton's second law at very low acceleration. However, some astronomical observations, especially the collision of galaxy clusters, contradict this theory. As we know today, the gas makes up most of the mass of a galaxy cluster. So when we see a cluster of galaxies we observe in reality the gas that composes the cluster, more than the stars in the clusters themselves.
The critical test of the theory of MOND is the collision of two clusters of galaxies that pass through themselves without even encounter because clusters of galaxies are dust grains in an empty vastness.
Here is the contradiction in the MOND theory.
In the theory of modified gravity, there is no dark matter, so let us imagine a collision between two clusters (top image). If there were no dark matter, the dominant matter of the cluster is gas. The collision of two clusters that pass through, we would show one side of the image, the ordinary matter that is to say the groups of stars of one of two clusters, and other side of the image, groups of stars of the other clusters. In the middle, we observe, between the two groups of galaxies, the accumulated gas in each cluster. Indeed the gas cloud of one cluster will interact with other cloud of the other clusters and will mix unlike the heavy matter (stars).
In the first case without dark matter (top), where most of the mass after the collision?
The mass is found in the central region, in the middle of the gas, since all of the gas is more massive than the total stars.
Now imagine the same experiment (bottom) where there is no gravity modified but the presence of dark matter. This time the dominant matter is the dark matter is that it focuses the majority of the mass, since dark matter is five times more massive than ordinary matter (stars and gas). On the second image, the bottom, dark matter is represented by gray bubbles.
When two clusters collide galaxies pass through, and the gas accumulates as in the previous example. And heavy dark matter will behave like ordinary matter of stars, it goes very little to interact and cross over the vastness of empty space to meet up with groups of stars on either side of the gas.
In the latter case the presence of dark matter, where is most of the mass after the collision? On either side of the gas where there is dark matter and not where there is gas as in the first case. We can see in this experiment that the presence of dark matter, or absence, as in MOND, gives different observations.
Proof of the presence of dark matter was brought by the observation of the cluster of Bullet.
By observing this cluster cosmologists come to rebuild the map of masses and observed that the masses are there where the galaxies and not where there is gas. This contradiction was resolved in the theory admitting the existence of a certain amount of dark matter in the form of neutrinos. Most of our universe is invisible.
Image: After collision of two clusters, in theory MOND, the mass is located in the gas cloud at the center (top). With dark matter, the mass is located in the gray bubbles (bottom). Credit image: Conference April 5, 2011 by Nathalie Palanque-Delabrouille astrophysicist at the CEA (Saclay).