What is Dark Matter?

What is Dark Matter?

Astronomy is such a progressive field that we make new discoveries every day. And thanks to astronomy that know how small we are in this universe, I mean we on an average are just 6 feet tall creatures on a huge planet which is one of the eight planets which revolve around a star which is one of billions in our galaxy and on and on, but astronomers are not satisfied of making us feel so small so they decided lets make it worst, so thanks to them that we know that the whole universe that is we have ever seen isn’t what everything that there is, we see things that we have ever observed are just 5% of matter in the whole universe. So let’s shed some light on these dark maters.

In 1960s-70s astronomer Vera Rubin was observing spiral galaxies and the topic of her interest was how the spiral galaxies traverse the universe and how they rotate, prior to her discovery she thought that the nebulae which are present in the galaxy, would revolve around the center of the galaxy slower as the distance between the center and the nebula increases. This assumption was based on Johannes Kepler’s observations that he made in 1600s, that the farther a planet is from the Sun the slower it revolves around it. Then Isaac Newton applied math to it and proposed that the strength Sun’s gravity can be measured which in turn gave us it’s mass, same was applicable with the galaxies if we calculated how fast a galaxy rotates and how the nebulae revolves around it, but nebulae are so huge that we can’t see them physically move but we could measure their Doppler shift which gave us it’s velocity. But the results were totally opposite of  what Rubin expected, for most of galaxies the farther the nebulae was from the center of the center of the galaxy the faster it moved.

This was totally bizarre as the results suggested that the mass was uniformly distributed across the galaxy, but the images of the galaxies clearly show that the number of objects at the outer of the galaxies decrease in number hence mass distribution of the galaxy should decrease from the center of the galaxy. The only explanation was that there was an invisible material which contributed for the mysterious mass distribution. Rubin found that there must be five or six times as much of this mysterious material than the visible matter in the galaxies. Back in the 1930s professor Fritz Zwicky had drawn a similar conclusion when he studied the speed of galaxies in galaxy clusters, he observed that the galaxies moved to quickly so much so that they should have left the cluster as there was not much gravitational energy to make them stay in the cluster. He stated that there must much more gravity in the clusters than what is present due to the mass of visible matter, but his observations had way more uncertainty in them and he hugely overestimated the amount of invisible material while Rubin’s observation much precise, but what stuck was the name given to the invisible material by Zwicky, he named it Dark Matter.

Scientist tried to explain that the mysterious mass consisted of dead stars, cold gas, loner planets, etc. But when they started cross examining them they found out nothing, like cold gas gives radio signals but nothing was found similar was the story was with everything else slowly each assumption was struck of their list even electron and neutrons and only one thing was left were those sub atomic particles, one such particle is called Axion. These particles have never been detected but their properties match to that of dark matter, e.g. they have mass, they don’t emit light so they are invisible to our naked eyes and the most interesting and spooky of them all, they don’t interact with matter, so they would pass right through you and you won’t even notice.

So how do we detect it, well the answer is Gravitational Lensing, it is a phenomena that allows the light to bend around objects with heavy mass, let’s take it this way a galaxy has a huge mass so it produces a great amount of gravitational pull, but how light can bend well according to Einstein’s theory light can behave as particle and as wave and as we know particles change they linear path to circular when exposed to gravity. So due to this we can see a galaxy behind another galaxy as the light from it is bent by the mass of the galaxy in front of it, this causes weird patterns for the observer. Thanks to Einstein’s equation lets us measure the mass of the galaxy in the front by use of the distortion of the light emitted object behind it. So all this hassle gives us a map of where the mass is located, astronomers used this method to locate a cluster of galaxies 3.5 billion light years away, called the Bullet Cluster. It’s not just a cluster but a collision of two huge clusters, which will merge to form a Mega Cluster, when these cluster collide the chance of galaxies colliding into each other is less as there is huge amount of distance between them example Andromeda the nearest galaxy to us is 2.5 million light years away. But what is interesting is that the gasses in the clusters collide into each other getting so hot that they emit X-rays.

So how do we detect it?

Astronomers at Chandra X-ray observatory map out where the hot gas was. And as expected it lies mostly between the galaxies having slowed down by the collision and as you can see in the photograph a bow shock shape has been formed in one cluster. But there as hundreds of galaxies farther away whose light is bent by the bullet cluster, by carefully calculating we calculated the whole mass of the Bullet Cluster and the dark matter in it. And if dark matter is made up of Axions they would pass right through each other, and when astronomers made the map that’s what they saw that there was a lot of matter shown in violet centered on the two clusters but it’s not the hot gas seen by Chandra as it is shown in Red.


We still don’t know what dark matter exactly is and Axions are just one possibility. Numerous experiments are conducted to study it but we are not that close yet, this makes dark matter much more mysterious than Anti-matter. I’m truly sorry for leaving Gravitational Lensing so vague as it is totally a whole new topic of discussion, but I assure you that shall see a blog about from us soon.