Does Dark Matter Really Matter?

What is dark matter? Does it really exist? Why is it important to study dark matter?
These are some of the questions I try to answer in this article. This is mainly a compilation of the research that I have done regarding the subject and my own understanding so far.

The term dark matter is actually a placeholder for this “invisible” matter that we hypothesise exists in the universe. It does not interact with the electromagnetic spectrum (so it does not absorb or emit light rays or any other form of energy), interacts with ordinary matter only through gravitational forces, and is present all around us.

It turns out that the total mass of ordinary matter (called baryonic matter) does add up to the total calculated mass of the universe. This is why we believe in the existence of dark matter that cannot be detected by us but still stands to account for some of the “missing” matter in the universe. The amount of dark matter in the universe is around five times the amount of baryonic matter.

One of the pieces of evidence that points out the existence of dark matter is the relationship between the distance of a star from the centre of the galaxy and the velocity of the star. We would think that because the centre of a galaxy is the brightest, it has the highest mass concentration, and thus, the stars on the inside move faster than those at the edges of the galaxy (the gravitational pull by the masses in the centre decreases as the distance of stars from the centre increases).
But, it turns out that the stars located on the outside move at speeds higher than predicted which indicates that there is some mass that we are unable to see, that is present throughout the galaxy, which provides the required gravitational force for stars at the edges to move with the speeds we observe.

Graph. (Modified from original image by PhilHibbs and licensed under CC BY-SA 3.0).

Another point that should be noted is- due to its speed, the stars on the outside of a galaxy should actually be thrown off into outer space, but this does not happen because the dark matter, which has a gravitational field associated with it, holds the stars together. In this sense, dark matter acts as a cosmic glue or as the connective tissue of the universe.

Yet another piece of evidence for the existence of dark matter can be found by studying gravitational lensing which is the idea that the fabric of space around objects is affected by their mass. What this means is that when light travels through a large distance, if a heavy mass is in its path, distortion ie bending of light can occur due to the gravitational pull from the mass. The amount of bending will depend on how heavy the mass is (more the mass of the object more is the bending due to more gravitational pull). It turns out that this bending of light has occurred in places where we cannot really see the matter that causes the bending, once again pointing towards the existence of dark matter.

Why dark matter really matters is because it played a key role in the formation of life and, is also important when we talk about ways in which the universe could possibly end.

During the formation of the universe, when the big bang occurred, the universe expanded on itself, and particles combined to form some of the fundamental particles that make the universe. The fundamental particles later combined to form stars which exploded to eject heavier elements. These heavier elements needed to then combine to form organic matter and rocky terrains necessary for life. It is believed that this “combining” only occurred due to the immense additional gravitational pull provided by dark matter. Without the dark matter, the clumping of these heavier particles into useful structures would not have occurred, which would have kept the universe homogeneous and even.

To understand one of the ways the universe could cease to exist, it is important to understand the term critical density which, is basically the average density of matter which should be maintained so that after an infinite time, the universe stops its expansion.

If the density of the universe is lower than the critical density(density=mass/volume), what it means is that we have less mass (in a fixed volume) thus leading to an insufficient amount of gravity. This in turn means that the expansion of the universe will never halt, leading to the creation of an open universe that will freeze over time.
If however, the density of the universe is greater than the critical density, meaning more mass (for a fixed volume), the excess gravity would lead to the creation of a closed universe which would collapse into itself.
This means that maintaining the critical density of the universe is crucial. This can only happen if all the matter including baryonic, dark matter, etc is in the right amount.
So in understanding a possible ending to the universe it is important to account for the dark matter that might exist in it.

To conclude, I think it is important to highlight the fact that dark matter is an extremely confusing and complicated subject that we know very little about. We still have a long way to go before we completely understand the ins and outs of dark matter but the little that we do know about it is extremely interesting to research and write about.

Drawing inspired by a simulation of dark matter done by NASA