Dark matter is one of the hot topics physicists are currently working on. The mysterious nature of dark matter makes it one of the most exciting subjects of modern day physics. Dark matter is not an ordinary matter. To be honest, nobody has a clue what dark matter is and what does it made of. Unlike ordinary matter dark matter does not interact with the electromagnetic force. It does not absorb, emit or reflect the electromagnetic waves. Out of four fundamental forces present in the universe, dark matter only interact with gravity for other three it does not give a shit! Its presence can only be felt by its gravitational effects on the objects.
Lets us walk though a simple example. The light (electromagnetic wave) come in touch with an object (made of ordinary matter like you, me etc) and we get to know the presence of an object. But in case of dark matter the light just passes through it, for light it does not exist. Now, imagine, what will happen if you want to pass through a wall made of concrete (ordinary matter). Of course you will break your head, but why could not you pass through it? The answer is because when you come in contact of the wall the matter of you body and matter of the wall exert an electromagnetic force on each which resist you to pass through it. Now switch the wall with dark matter, what if the wall is made of dark matter? Yes, you won't break your head this time. You will pass through it just like it is never there. Like in old ghost movies ghosts are passing through the walls and the person passes through the ghosts and can't see them. Ironically, the ’dark’ in the name ’Dark Matter’ does not indicate that the matter is dark but it suggests that the matter is so transparent that it becomes invisible to us and its nature remains mysterious.
Interesting matter, right? but why do we care about it? Presence of dark matter is in abundance. Understanding dark matter is significant to determine the shape, size, and future of our universe. The nature and the amount of dark matter present in our universe has important role in determining the fate of our universe such as whether the universe will be ever expanding, or it expand up to a limit and then it will collapse or will it expand up to a limit then remain in that state after reaching equilibrium. Dark matter is also crucial for understanding the evolution of galaxies. As galaxies rotate about their axis their rotation should break them apart. But this does not happen galaxies seem to have enough gravity to hold the stars in the galaxies together. How does galaxies get this gravity if there is not enough visible matter for it? Presence of dark matter gives enough gravity to hold the stars together in the galaxies. If we add up the masses of all the galaxies, black holes, neutron stars etc, everything which is present in the universe that amount of mass is deficient by a large amount of mass needed to have gravity that gives the universe and the galaxies the structure it presently has. Simply, the universe is governed by the gravity which is much larger and it cannot be produced by adding all the visible matter mass. In 1933, Zwicky, showed that the mass of the Coma Cluster by using a virial theorem and compared with the mass calculated using the brightness and the number of galaxies in the cluster. What he found out was astonishing. The total mass calculated by the virial theorem was approximately 400 times larger than the mass calculated by other methods. This work was the first indication of presence of dark matter. He predicted that there must be a large amount of hidden non-luminous matter contained in the cluster, and he called the matter ’Dark Matter’. Following this work, in the 1960’s more evidences was provided by Rubin, Freeman and Ford in the support of the presence of dark matter. With great accuracy they measured the velocity curve of edge-on spiral galaxies. They estimated that galaxies must have 6 times more dark mass than the visible ordinary mass. This result was firm evidence for the presence of dark matter and caught the attention amongst the science community. Today, we have more accurate data and the energy density of the universe it is measured to be 24% dark matter and 4% ordinary matter.
Around 85% of matter of the universe is in the form of dark matter, that means until now we are only playing and trying to understand the 15% matter (visible matter) of the universe since centuries (do not have complete theory to explain this 15% even). Who knows this 85% of matter may be not made of particles, does not have a wave nature. We have something new, but definitely something unimaginable to come across.
