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**The dark matter model. **
In our own galaxy we have an evident missing mass effect. Three or five times more mass should be necessary to balance centrifugal force. Our sun, orbiting at 230 km/s around the center of the galaxy, should have gone away, with its planets, since millions of years. The situation is worse for clusters of galaxies, that should "evaporate", for the individual proper velocities of the galaxies are higher than the escape velocity of the cluster.
You perhaps know that the circular orbital velocity of a satellite, around the Earth, is close to 9 km/s. The escape velocity is 11 km/s. We need to give a space station a velocity higher than 11 km/s if one wants it to escape Earth's attraction and visit distant planets, like Jupiter or Saturn.
Suppose we would observe satellites orbiting around the Earth at 20 km/s. We should conclude that the mass of the Earth has been underestimated. In the 30's, Fritz Zwicky pointed out this for galaxies and clusters of galaxies, from velocity measurement ( Doppler effect ) and mass evaluation ( photometry ) This problem is still embarrassing. How to manage it ?
The today's classical answer is called "dark matter". People think that a large amount of hidden mass exists in galaxies and clusters. Their argument is the following. Galaxies produce gravitational mirages : multiple images of distant quasars. Clusters of galaxies bend space and produce other mirages : multiple images of galaxies. The space telescope brought many, now. The observed mass is too weak to explain such strong effects. So that the astronomers consider this as a proof of the existence of invisible dark matter. Some say that the gravitational lensing is a new way to "weight" galaxies and clusters. Others have started to map the universe, basing their design on this gravitational lensing effect.
There are mass concentrations in the planets especially in the Moon. We call it "Mascons". When a satellite flies around the moon, we can record its path very precisely, with a laser. This path is not perfectly circular. It is modified by underground mass concentrations. Some suppose these mass concentration correspond to ancient impact of heavy huge bodies. Doing that, we found the mass mapping of the Moon on measurements of the geodesics around it.
Similarly we could say that gravitational lensing refers to geodesics (light's path). Then the analysis of such geodesics may be a method to evaluate hidden mascons in galaxies and clusters of galaxies. In our own works you will find an alternative interpretation of such strong lensing effects. We think they mainly could be due to negative lensing, caused by surrounding "ghost matter".
Returning to the classical interpretation through dark matter, what could it be made of ? Astronomers think about MACHOs ( MAssive Compact Halo Objects ). Possibly : faint stars, too faint to be detected. Detection method : occultation. Stars are very small objects. A galaxy, considered as a gas whose stars would be "molecules" is a very rarefied gas. So that the chance that a star masks another one is very weak. But there are many. The sky survey can be managed by computers. They at last can analyze the light emission variation of a great number of stars, automatically. Some are due to known binary systems. Methods make it possible to discriminate between binary systems effect and distant occultation by "dark stars", machos.
Results : disappointing.
Another candidate : exotic matter, like hypothetical massive neutrinos. Not confirmed at the present day. The French astrophysicist François Combe suggested that large amounts of very cold hydrogen could exist, in the Universe. So cold that it could not be detected. She used this invisible hydrogen to "explain" the aspect of the rotation curve of galaxies ( large peripheral circular velocities ) and to produce (transient) spiral structures. Dark matter is invoked for everything. Some use it to "explain" the VLS, the Very large structure of the Universe. Many journals publish such works ( Astrophysical Journal, Astronomy and Astrophysics, and so on...), apparently without any state of mind, as a "natural approach". Like black holes, dark matter does exist, in the pages of journals, I think. In this cosmic hypermarket you can choose hot or cold dark matter, dwarf or giant black holes, as you like.
Strange epoch. Before the Italian Torricelli's discovery (1640) : the discovery of atmospheric pressure, people believed that mercury climbed in barometers because its horror of the void.
Nature hates void.....
Before Torricelli, people measured the level of void's hate.
(100)
**- But... I don't see anything : It's all black !
- Normal. It's dark matter's map. **
For publishing managers and referees, some speculations are arbitrarily permissible, some others, no.