About the celestial objects...

Dark matter and its effects on the universe
Dark matter (also called non-baryonic, invisible or non luminous matter) is a mysterious matter that does not interact with electromagnetism or itself but through gravity. All matter (including dark matter) follows the formula density is proportionnal to aE-3. It is not to be mixed up with antimatter with whom it does not share any property. This non baryonic-matter is found around large systems and bodies, forming halo around galaxies (the dark matter halo around the milky is thought to be at 100kpc from the central bulge). It is distributed evenly among large scales. Non-visible matter is thought to make up 27% of our universe and has a 1 to 6 ratio amount of mass compared to visible matter. Scientist have conducted dozens of experiments trying to find evidence for this matter. The main candidates are:

Massive Compact Halo Objects theory (Machos) which state that dark matter is baryonic matter stuck around it and outshined by other objects

Weakly Interacting Massive Particles (WIMPs)

Axions

Other particles like neutrinos

The different kinds of dark matter:

Cold dark matter (CDM): with a low velocity compared to c, it plays an important role the formation of galaxies as we know them today.

Hot dark matter (HTM): with its velocity being a significant percentage of c, it smooths out the large-scaled structures

Warm dark matter: a mix of hot and cold dark matter

History of dark matter
The credit of the discovery of dark matter goes to Fritz Zwicky (a switzt physicist who studied the coma cluster). He first found that the equation describing gravity did not acount for the large amount of matter (which he called dark matter) in the cluster. His conclusion was that there was 400 more times mass than what is visible. Even though, his calculation was off by a factor of 70 (the actual ration being 1:6), the main point was that there is more unobservable matter than there is of baryonic matter. However a few people discovered similar results before him whereas the scientific community did not find amplereason to take his work seriously (ie. Kelvin, or Poincare). In 1939, scientist performed calculation on Andromeda's outer branches rotational velocity. Their conclusion was that they were fasting much faster than gravity and the centrepical force would allow. They did not think that those results could be explained by dark matter until Oort connected it to Zwicky's discovering. While some more calculation was performed on other galaxy, showing the same kind of "paradox", more evidence for the non-visible matter was found using gravitational lensing.

Evidence for dark matter

The lambda-CDM model
The lambda-CDM model is the current model we have to explain the big-bang. "lambda" accounts for the cosmological constant (baryonic matter) and the dark energy while CDM stand for cold dark matter. This model provides an acccurate explication for the following phenomena (and summarizes well the different evidence for dark matter)

the cosmic microwave background

large scaled structures (clusters of galaxies)

the abundance of helium, hydrogen and lithium

the accelerating rate of expansion of the universe

The Cosmic Microwave Background

The cosmic microwave background is a representation of the universe in its early stages. The slight variation in the mass distribution (caused by an oscillation: collapse and pushback from radiation) lead to slightly cooler and hotter region (0.01% of difference). The scientist then plotted the pattern of this representation on a graph (fig1.). It is shown that we would need 5 times more dark matter than there is of baryonic matter for fig2. to match the fig1.

Large structures (galaxies and clusters)

Gravitatitional lensing occurs when dark matter (gravitational lens) causes sufficient curvatures of for the path around it to be visibly bent, as if by a lens. This phenomena explained by Eintstein theory of relativity gives the impression that the galaxy or quasar appears several times in the cosmic background.

The formation of large baryonic matter structure (part of the cosmic web) is made possible by the existence of dark matter (HDM). Without the later, those big scaled structures would be inexistant or far simpler. In a universe without dark matter the gravitational pull would be a lot less important. We can therefor see dark matter as the skeleton of the universe.

Another evidence we have for dark matter is the bullet cluster. When the 2 clusters collided the interstellar gas got stuck in the middle. Dark matter was able to pass through the gas. whereas baryonic matter got stuck on either side The point where the centre of mass should be with baryonic matter does not match the experimental results

The rotational velocity of galaxies

According to Keplers second law, galaxies arm should slow their rotation as they move outwards in order for the mass their not be ejected by the centripetal force (blue-dashed line). On the other hand, because dark matter is in a halo around the galaxy, therefor adding mass to the outer parts of the arm, the rotational velocity of galaxies is actually the same everywhere (red line on the graph).

The DAMA/LIBRA experiment

In Italy, scientist have detected an annual cycle of the rate of detection... This could potentially be the first evidence for dark matter Could this be that the earth is moving “with” dark matter half of the year (orbiting the sun with a rotational speed of 220km/s around the central bulge) and “against” dark matter flow for the rest of the year? Could this be a completely random climate factor due to the seasons, the humidity, the atmosphere... This is the reason why scientist are curently conducting the same experiment in Melbourne (southern hemisphere where the season are actually reversed).

If the results are the same, this would be the first evidence of dark matter ; if not it was only a false alarm. Scientist are trying to find evidence for WIMPS. If dark matter hit an atom (sodium ionide), there will be a photon emitted which is multiplied by the photomultiplier tubes. Trying to avoid false alarms,scientist are performing the experiment deeply undergroung to avoid "particle pollution". There is also a muon detector (for cosmic rays) and the soidum ionide is surronded by liquid alkybenzyme which emits a photon when absorbing gamma rays (to avoid false detection of atom decomposition, even tough those decomposition are very rare with this kind of material) We are waiting for the results...

Links to other interesting article on the subject: