What is Dark Matter?

 The universe we see was never the same as it is today. It came into existence just some billion years ago before it was like a hot soup of electrons and protons. The whole mass and energy were concentrated in a point, very dense. And all of a sudden, there happened a Big Bang. All the matter started expanding in space, and this universe came into being. 

This universe is all here present and stable due to five forces, the fundamental forces of nature.

1. The electric force
2. Magnetic force
3. Weak Nuclear force
4. Strong Nuclear force
5. Gravity

The electric force we all know is between static charges. It is responsible for atomic bonding in molecules.

The magnetic force is between magnets or moving charge particles. Electric and magnetic forces together form electromagnetic forces, which generate an electromagnetic field. Light, heat, and most of the energy in this universe travel in the form of electromagnetic waves.

The Weak Nuclear Force holds the quarks in protons and neutrons.

The Strong Nuclear Force holds protons in the nucleus of an atom.

And finally, gravity is responsible for the shape and motion of galaxies, the solar system, and other celestial systems.

All these forces are not on their own. There are careers of these forces. Photons are the carrier of electromagnetic forces, Gluons are the carriers of strong-nuclear forces, Bosson is the carrier of weak-nuclear energy, and graviton is the carrier of gravitational force. Among these fundamental particles, only graviton is hypothetical. There is no physical particle yet found of this force.

In the late 1600s, Sir Issac Newton told us that gravity is a universal force, an intrinsic property of every mass body.

In 1915, Einstein told us that gravity is a bend in the space-time fabric. The greater the density, the greater the gravity.

Everything was going fine until 1933 when Swiss astrophysicist Fritz Zwicky studied galaxies in a comma cluster and estimated that there was 400 times more mass than the visually observable.

His calculations needed to be more accurate due to the obsolete value of Hubble's constant. But later in 1970, an American astronomer, Vera Rubin was studying spiral galaxies and calculated the velocity curve of edge-on spiral galaxies with greater accuracy. 

She found that the galaxies were rotating with a velocity more than expected. An influential paper presented Rubin and Ford's results in 1980. They showed galaxies must contain about six times as much dark as visible mass; thus, by around 1980, the apparent need for dark matter was widely recognized as a major unsolved problem in astrophysics.

As the name suggests, dark matter is not dark or black in color. We call it dark because it does not interact with ordinary light and we cannot see it. But there are methods to detect this dark matter; one method is gravitational lensing. Whenever light passes nearby a massive body of immense mass light bends around it, this bending of light is called "Gravitational Lensing".

Two views from Hubble of the massive galaxy cluster Cl 0024+17 (ZwCl 0024+1652) are shown. To the left is the view in visible light with odd-looking blue arcs appearing among the yellowish galaxies. These are the magnified and distorted images of galaxies located far behind the cluster. Their light is bent and amplified by the immense gravity of the cluster in a process called gravitational lensing. To the right, a blue shading has been added to indicate the location of an invisible material called dark matter that is mathematically required to account for the nature and placement of the gravitationally lensed galaxies that are seen.

Credits: NASA, ESA, M.J. Jee and H. Ford (Johns Hopkins University)

To date, there is no practical evidence of dark matter. we have not yet found any dark matter particles that we call WIMP (weakly interacting mass particles). WIMPs are hypothetical particles that are considered to be responsible for dark matter. Research is going on to find these particles, let's hope for the best.