The universe is not homogeneous, it has small imperfections, so called, unhomogeneities of density. It is they who do that the stars exist, that galaxies exist, we exist. These unhomogeneities of density were created by gravitational collapse and allowed the matter to exist inside larger structures.
One of the major problems in modern astrophysics is that we do not know the nature of the essential components of the Universe. The luminous matter, the only one we see directly, appears to be less than a tenth of the mass of the universe. We must be creative because this light, from radio waves to gamma rays, is our only source of information. Whenever our telescopes send us images of the universe we seek to uncover the invisible entities that maintain its structure. We now know that our universe is not composed only of atoms, 96% of our Universe was reported missing at in our telescopes. Since there is no question of challenging the laws of gravity, all scientists agree, it lacks mass in the observable universe to explain its stability. Part of this missing mass is matter, as we do not see it was called dark matter. Although the luminous matter is a minority, we see many objects, planets, stars, galaxies, clusters and especially very large areas of gas. And this light from these objects, we allows understand invisible phenomena.
For example, when objects (planets, stars, group, group of galaxies, clusters of group), revolve around a central point, their distance from the center and speed allows us to calculate the total weight of components . Thus it has been noted that the velocities of stars orbiting the galactic center does not decrease with their distance from the center but continue to be constant. This observation is in contradiction with the laws of gravitation. The only explanation is that the present mass of the galaxy continues to grow gradually in proportion as one moves away from the center of the galaxy.
And of course, it is the same for large structures like galaxies and clusters of galaxies. Today our detectors are able in certain wavelength, to show us that the galaxies and clusters are surrounded by a thick halo where lies most of the matter, which is called dark matter.
Particle physics describes the properties of matter and teaches us that ordinary matter, called baryon is formed of quarks (protons, neutrons) and leptons (electrons, neutrinos). So the baryonic matter, that we see, is made of atoms which we find protons and neutrons surrounded by a cloud of electrons. But the matter exists also in other forms, ionized form called plasma, in the form of atomic elements (hydrogen, helium, carbon, nitrogen) in the form of atomic clusters (nanoparticles) in molecular form (dust) under vacuum energy (virtual particles) and may take the form of exotic matter such as WIMPs (hypothetical massive particles) form. We know that most of the ordinary matter in the universe is composed of hydrogen and helium, but we do not know what dark matter is formed.
NB: In astrophysics, the WIMPs (Weakly interacting massive particles) are weakly interacting massive particles. These hypothetical particles are a solution to the problem of dark matter. These particles interact very weakly with ordinary matter (nucleons, electrons). This very weak interaction, associated with a large mass (of the order of that of an atomic nucleus), make a credible candidate for dark matter.
NB: An atomic aggregate (cluster), is a set of atoms sufficiently narrow to have specific properties, and intermediate in size between a molecule and a solid massive. Clusters apply to all types of atoms involved in the structures of several thousand atoms as nanoparticles.
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Image: The universe is not homogeneous, it has small imperfections, so called, inhomogeneities of density. It is they who do that the stars exist, that galaxies exist, we exist. Analyses of the sky by the WMAP (Wilkinson Microwave Anisotropy Probe) probe, indicate that the universe is old 13.82 billion years (with an accuracy of 1%), it is composed of 73% dark energy, 23% of cold dark matter, and only 4% ordinary matter (atoms). The universe is currently expanding at the rate of 71 km / s / Mpc (with an accuracy of 5%). The observable universe went through episodes of rapid expansion called inflation and will grow forever. Credit: WMAP Science Team, NASA
Clusters of galaxies are not formed, than to galaxies, they bathe in cold low density gas (1000 particles/m3) and in the extremely hot gas (10 to 100 million degrees). At these temperatures, the gas is fully ionized, it is a visible plasma in the field of x-rays. The gas is distributed in a way, much more diffuse, it fills the space between the galaxies and extends well beyond. The mass of gas belonging to the galaxy is much larger than the mass of the galaxy itself. If we measure the gravitational dynamics of the universe at large scale, the mass of ordinary matter in the observable universe is only 4% of the total mass. 23% of the mass is dark matter and 73% of dark energy. This is described in a predominantly accepted model, the SCDM model (Standard Cold Dark Matter). What we see when we look at the light of stars, galaxies and clusters is ordinary matter.
But how can we see dark matter?
Clusters of galaxies are the largest observable structures of matter. They consist of hundreds of galaxies bound together by their own gravitational attraction. Ordinary matter of galaxies is mainly gas, because the mass of the gas is much larger than the total mass of stars. All matter, ordinary matter and dark matter undergoes gravitational forces. It is in the Bullet cluster, that the cosmologists could "see" dark matter. The Bullet cluster or 1E 0657-56 (Bullet cluster) observable in the constellation Carina, is the result of the collision of two clusters of galaxies that happened there 150 million years. The study of this collision began in August 2006 and showed one of the strongest proofs of the existence of dark matter. When clusters or galaxies collide, the matter (stars, gas and dust) is perturbed by the gravitational forces. In reality, heavy objects like stars do not collide, they pass, one next to the other without ever meeting, because the space between the stars is immense. The stars are therefore not affected by the collision, they can be accelerated or slowed slightly gravitationally but not destroyed. By cons during the collision, the cold and hot gases that constitute the bulk of the baryonic mass of galaxies, will interact with each other, they will even be strongly and quickly slowed. They will mix more easily because of their atomic freedom and their very weak bond.
This is what we see on the composite image below cons. This gigantic collision between two clusters generated considerable energy, perhaps the most powerful of the universe since Big Bang. It is in the domain of X-rays, the observation of the collision sheds new light on dark matter, because the stars, gas and dark matter behave differently during the collision.
Galaxies of two clusters of galaxies are observed in visible light, it is the white spots, the hot gases of the two clusters are observed in X-rays, which are the red clouds, dark matter is shown in blue.
But what do we see exactly?
We see the result of a collision between two clusters. In this picture there are hundreds of galaxies grouped together in clusters but mostly we see a small cluster of galaxies in the blue right spot and a large cluster of galaxies in the blue spot on the left. Both gaseous envelopes of the two clusters are in red color, small red spot follows the little blue spot and the great red spot follows the big blue spot. In fact the small cluster of galaxies right, just cross the great cluster left. The huge collision "disheveled" two clusters of their halo gas causing a shock wave visible in the tip of the little red spot. This shock wave strongly compressed and therefore heated the gas in the cluster to the point to reach 100 million degrees. The cluster of the bullet is one of the hottest clusters known. In places the telescope Chandra X-ray Observatory has measured a speed of gas 4500 km / s.
The two clusters are now separated by 3.4 light years and the total mass calculated according to their speed and distance, represent much more than the mass of the visible ordinary matter (galaxies seen in the optical and gas seen in the X-rays). These are deliberately colored blue areas that show the distribution of the invisible dark matter in the cluster. In the frontal impact colossal, dark matter behaved like ordinary matter, it did not interact, it crossed another dark matter smoothly while the interstellar gas was snatched of the clusters. This caused the shock wave that can be seen in the red bullet-shaped cloud of gas right. The clear separation of dark matter and gas clouds is considered as direct evidence of the existence of dark matter.
Image: What do we see in this false-color composite image?
We see all the matter in the cluster of the bullet "bullet cluster." Located 3.4 light years away from each other, the two clusters of individual galaxies Bullet are in the blue area and the two galactic gas clouds are seen in X-rays, in red). The blue colored areas represent the bulk of the mass of clusters, i.e. dark matter, six times more massive than ordinary matter. The cluster of the bullet is the smaller of the two cluster that crossing through the other side to the other. The huge collision "disheveled" two cluster of their halo gas causing a shock wave visible in the tip of the little red spot. This shock wave strongly compressed and therefore heated the gas in the cluster to the point to reach 100 million degrees. It stands out as a bullet followed by his trail of gas.
Credit: X-ray: NASA/CXC/CfA/ M.Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al. Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.;