Dark Matter: An Unsolved Mystery

 

As the name suggests, dark matter is an invisible form of matter that is supposed to fill a substantial part of the universe and is responsible for driving some critical cosmological events. One thing is certain, it is not in the form of stars or planets that we can see around us. From the observations, it is estimated that 85 % of the total matter content of the universe must be dark matter. Along with dark energy, it forms 95 % of the total mass-energy content.

Why is it dark?

Dark matter seems to have no interactions with the electromagnetic field, which implies that it does not emit, reflect or absorb light. This is the reason why it is totally invisible to us and hence aptly termed dark matter.

Then how is it detected and what role does it play in the dynamics of the universe?

Observations from the evolution of galaxies suggest that the galaxy dynamics would be quite different if it is governed by the presence of the matter that we see in the universe (visible matter). The galaxy rotation curves (path of motion) would be quite different if gravitational interaction emerged from visible matter only. Many galaxies would never have formed if it was only due to the contributions of normal matter. Simply we need more matter to generate the necessary gravity such that the observed galaxy dynamics can be explained!! To put it in another way, we need around 27 % of matter to explain the observations, but the normal matter is far less than that.

So it is quite understandable that there is a missing link in the picture. Either our observations are faulty or there is some form of matter that is totally eluding our vision, but playing a significant role in the structure formation and evolution of galaxies. This invisible form of matter that generates sufficient gravitational interaction to sustain the galaxy dynamics is dark matter. We cannot see it, but we can feel its presence due to the role played by it. There are other scientific pieces of evidence of the presence of dark matter such as gravitational lensing (bending of light in a gravitational field), Cosmic microwave background radiation (which is a relic of the Big Bang), etc.

What is dark matter made of?  

It is now well known that dark matter has minimal interactions with visible matter & radiation, which is the basic reason behind its mysterious nature. It remains secluded without intermingling with other components of the universe but reveals its presence only through gravitational interaction. So naturally, a question arises regarding the chemical composition of dark matter. In simpler terms, one would like to know what is dark matter made of. This question is not yet answered by the scientific community, but extensive research is underway. It is believed that dark matter is made up of some new kind of elementary particle that is yet to be discovered. 

 

Speculations:

Dark matter is sometimes argued to be some form of antimatter (matter composed of antiparticles of the corresponding particles in the ordinary matter), but this is not correct. The reason is, that we do not see the unique gamma rays that are supposed to form when matter annihilates with antimatter. Some people argue that dark matter can be black holes, which are themselves mysterious invisible objects, and share some common properties. But this is not true either, because black holes are centers of gravitational lensing due to their enormous gravitational pull. If dark matter was present in the form of black holes we would have seen far more gravitational lensing phenomena than we actually see in the universe. As of now, we believe that dark matter must be made up of some exotic particles which are yet to find their places in the periodic table and, about which our knowledge is extremely limited.

By Prabir Rudra

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