Dark matter is an intended form of matter that does not emit or reflect electromagnetic radiation and therefore cannot be observed in the normal way. Dark matter can be detected only indirectly through its gravitational influence on ordinary matter, or by its weak interaction with the latter. The concept of dark matter was introduced as an explanation for why galaxies rotate much faster than they should be doing in relation to the amount of visible matter they contain and why galaxy clusters do not move as they should if only visible matter existed. Nowadays there are also many other observational evidence for the existence of dark matter, but they do not allow to understand what it consists of. This is one of the great unsolved problems in physics.
Free example dark matter research paper will let you understand that researchers have measurements allowing to conclude that the total energy of the universe (which according to E = mc² is the same thing as mass) consists for 23 percent of dark matter, and only 5 percent of normal matter. The remaining 72 percent consists of dark energy.
What dark matter is made up of is still a mystery. The two most common explanations are MACHO (Massive Compact Halo Objects) and WIMP (Weakly Interacting Massive Particles). MACHO are thought to be macroscopic objects like brown dwarfs (Jupiter-like bodies having not enough mass to become stars). WIMP are thought to be elementary particles of exotic matter, with large mass but with only weak interactions with the normal matter except for the gravitational interaction. Supersymmetric particles are an example of theoretically proposed exotic matter. Neutrinos were also long among the large number of candidates, which could be explained the observed gravitational effects. Previously, they had been considered massless but the new findings suggest they had mass, albeit small. Researches have now managed to fix the limit on how large the three neutrinos mass may be and it turns out that despite their large numbers can only give a small contribution to the total mass of dark matter. Recent experimental results also show that MACHO just can at most explain a small part of the dark matter and the WIMP are therefore the preferred option.
There are also other options such as the original black holes, gravitinos (hypothetical supersymmetric partners of gravitons), and axioner, which is another type of exotic particles.
In August 2006, researchers reported that, among other things, using the NASA’s Chandra Telescope and the Hubble Space Telescope let them see direct evidence of dark matter in a collision of two galaxy clusters, where the ordinary matter was separated from the dark matter. The system is called Bullet Cluster (1E 0657-56). By using gravitational lenses, they could see that the source of gravity of the clusters does not overlap with the visible matter, indicating that the bulk of the mass is in the form of dark matter . Another example of colliding galaxies with separated dark and ordinary matter, MACS J0025.4-1222, was found in 2008.
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