What is Dark Matter?

The name dark matter means that this form of matter is not visible because it does not interact with electromagnetic field. Since light is an electromagnetic radiation, dark matter does not absorb, reflect or emit light and hence, it is not visible.

Galaxy

We know about the presence of dark matter from its gravitational effects. In our universe, galaxies are rotating with very high speed. To hold all the masses in the galaxies together, there should be an immense amount of gravity otherwise they will get torn themselves apart. But, if we calculate the gravity produced by the observable matter, we find that this gravity is not enough to hold them together. This is also same with galaxies in clusters.

Currently accepted theories of gravity do not explain such astronomical observations. This leads scientists to believe that more matter is present that we cannot see. This yet to detect extra mass is giving these galaxies the required gravity to stay intact. If the galaxies did not contain a large amount of these unseen matter, then they would behave very differently. The formation of some galaxies would not have been possible and other galaxies would not move as they currently do. This unknown matter is known as dark matter.

Dark matter is believed to be abundant in the universe. So, dark matter must have had a strong influence on the structure and evolution of the universe. In the Lambda-CDM model of cosmology, the total mass energy content of the universe contains;

Mass Energy Content of the Universe
  • 68% Dark Energy
  • 27% Dark Matter
  • 5% Ordinary Matter and Energy

Hence, about 85% of the total matter in the universe is dark matter. And, dark matter and dark energy constitute 95% of the total mass energy content.

Evidences in support of dark matter

Other than the gravitational effects, there are also some other observations that prove the existence of dark matter. These observations include:

Gravitational Lensing

Gravitational Lensing is a phenomenon that occurs when a massive object (such as a galaxy cluster) causes a sufficient curvature of space-time for the path of light around it to be visibly bent. This phenomenon gives the evidence of the presence of large amount of dark matter in galaxies and clusters of galaxies.

Enough lensing events are not seen around objects like large galaxy sized black holes to make up the required 27% dark matter contribution. From this, we know that the required amount of gravity is also not produced by such massive objects.

Dark matter map of a patch of sky observed by Kilo-Degree Survey based on gravitational lensing
Dark matter map of a patch of sky observed by Kilo-Degree Survey based on gravitational lensing

Image Credit: Kilo-Degree Survey Collaboration/H. Hildebrandt & B. Giblin/ESO

Cosmic Microwave Background (CMB)

Dark matter does not directly interact with radiation, but its gravitational potential and its effects on the density and velocity of ordinary matter does effect the Cosmic Microwave Background (CMB).

The precise structure of the observed CMB angular power spectrum is well fitted by the Lambda-CDM model. Hence, CMB provides a powerful evidence in the support of dark matter.

Other Evidences

The motion and heat of gas, that gives rise to observed X-rays, also support the presence of dark matter in the centres of galaxies and clusters of galaxies. For example, in the Bullet cluster, there are two merging galaxy clusters. The Chandra X-ray observatory has observed that the hot gas (ordinary visible matter) is slowed by the drag effect of one cluster passing through the other. However, the mass of the clusters is not affected which indicates that most of the mass consists of dark matter.

Other than these, the formation and evolution of galaxies, the mass location during galactic collisions and the motion of galaxies within galaxy clusters also support the existence of dark matter.

Dark matter map by the Dark Energy Survey (DES) using weak gravitational lensing data set (2021)
Dark matter map by the Dark Energy Survey (DES) using weak gravitational lensing data set (2021)

Image Credit: Dark Energy Survey (DES)

Dark matter map by the Hyper Suprime-Cam Survey (HSCS) (2018)
Dark matter map by the Hyper Suprime-Cam Survey (HSCS) (2018)

Image Credit: Hyper Suprime-Cam Survey (HSCS)

Dark matter map by Kilo-Degree Survey (2015)

Image Credit: Kilo-Degree Survey (KiDS)


Although dark matter is yet to detect, there are still some possibilities that are viable. And, on that basis, dark matter has been divided into two varieties.

Baryonic Dark Matter

Baryonic dark matter is made up of baryons (protons, neutrons and atomic nuclei). But, dark matter is not in the form of dark clouds of baryons because if this was true, then we would have been able to detect those baryonic clouds by observing their interaction with radiation passing through them.

However, baryonic matter can still make up the dark matter if it is all bound up in brown dwarfs or in small, dense chunks of heavy elements. These possibilities are known as Massive Compact Halo Objects (MACHOs). This baryonic dark matter has been determined by measuring the abundance of elements heavier than hydrogen.

Non Baryonic Dark Matter

Most of the dark matter is thought to be non baryonic and is made up of other yet undiscovered particles.

Dark matter is classified as cold, warm or hot according to its free streaming length. Current theories and observations suggest that dark matter is relativity cold or non relativistic which indicates that they are made up of heavy slow moving particles. Also, these particles do not interact with electromagnetic field. Hence, these particles are known as Weakly Interacting Massive Particles (WIMPs).

Many experiments are done to detect and study the precise nature of WIMPs, but none have yet succeeded. They are not predicted by the standard model of particle physics. But, theories such as supersymmetry predict that hypothetical elementary particles (like axions) may be dark matter.


Alternative to Dark Matter

Although the existence of dark matter is widely accepted, many theories have also been proposed as an alternative to dark matter. Some astrophysicists argue for various modifications of the standard laws of general relativity. These include modified Newtonian dynamics, tension-vector-scalar gravity, or entropic gravity. Some of these modifications suggest that the attractive force exerted by ordinary matter may be enhanced in condition that occurs only on galactic scale.

But most of these theories are unable to give a satisfactory explanation. In the Bullet cluster, dark matter physically separated from ordinary matter has been observed. This separation is not explained by such theories and leads to the conclusion that dark matter is a physical reality.


References: Wikipedia, Britannica, NASA, CERN

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