What is Dark Matter and Everything we know about it?
By: Jatin Chopra
Tags: Astronomy, Science Communication, Astrophysics, Cosmology, Dark Matter

Universe 101

Image credit: ESO/WFI (Visible); MPIfR/ESO/APEX/A.Weiss et al. (Microwave); NASA/CXC/CfA/R.Kraft et al. (X-ray)

What comes to your mind when you try to ponder what makes up the universe? You might guess things like mobiles, laptops, Wi-Fis, cats, dogs, internet trolls, Instagram influencers, stars, planets, galaxies, black holes, and all that awesome stuff you saw on the Discovery channel! However, you are missing a whopping 95% of the whole of creation.

Almost all astrophysicists and cosmologists believe that all the things as mentioned earlier account only for 5% of the whole. The remaining 95% constitutes stuff we can not see! ( let that sink in for a moment)

Energy and Matter in the Universe

Astronomical surveys and observations show that from all of the Matter and energy in the cosmos 27% account for Dark Matter, 68% for Dark Energy, and the remaining 5% for Normal Matter. In hindsight, we must not name our matter “Normal Matter” seeing that it’s quite scarce in the universe.

Let’s consider the idea of Dark Matter first. Quite often people confuse Dark Matter with Dark energy. Two are different entities on their own. Dark energy is super awesome. It is the complete package, and I want to do justice to it, so it’s a story for another time. Let’s talk about Dark Matter and why astronomers are sure that it exists. Spoiler Alert! We have no clue! Dark Matter is literally present in your room right now, and countless astronomers and particle physicists are searching for it.

Discovery of Dark Matter

The jaws of many astronomers dropped when they observed that the stars in many galaxies are moving faster than expected. Stars in star clusters and galaxies in galaxies clusters show this behaviour too. The stars are moving so fast that they should have long left the galaxies that they are orbiting in, leaving no stars in galaxies. Thus everything is moving too fast to create and maintain any structure in the universe. But, that’s not the case. We have structures in the universe. There are galaxies with stars in them, stars making star clusters and galaxies creating beautiful galaxy clusters.

This is a simulation of galaxies without (on the left) and with (on the right) any Dark Matter. All the galaxies in the universe behave just like the right side galaxy. (Image credit Forbes.com)

Now, where does Dark Matter come in the picture? From the motion of the stars, we can get an idea about gravity. Consider the solar system. The planets that are closer to the Sun orbit faster than the planets in the outer solar system. The closer planets experience more gravity from the Sun than the worlds at the end.

Hence, Mercury takes only 88 Earth days to orbit once around the Sun while Neptune takes 165 Earth years to do so! So the stars that are at the exterior regions of the galaxies must orbit slower than the stars in the inner sections. However, to everyone’s surprise, those stars orbit with similar speeds that of the inner stars! This is impossible! The above means that the outer regions experience equal gravity that the internal areas do!

Given that the above is true, then there must be more matter to give us that extra gravity. But, any further observations fail to discover any extra mass. After failed attempts of detecting additional matter, many clever astrophysicists started to think that maybe that extra matter is invisible and coined it, Dark Matter!

Another discovery that advocated in favour of Dark Matter was the discovery of Gravitational Lensing caused by Dark Matter. According to Einstein’s General Relativity, light can bend around objects with high mass. To understand the above, consider a faraway star, which is studied well. Now, suppose a heavy object like another star or a planet happens to pass through our line of sight. Then, due to the gravity of the star/planet passing, the light from the distant star bends around the star/planet and produces two images of the same star.

To an observer, it appears as if there are two stars, where in fact, there’s only one. This is called Gravitational Lensing. Here the star/planet acts as a lens. It is one of the many beautiful and awesome consequences of General Relativity.

When light from a distant quasar passes by a large mass (such as a massive galaxy), its gravity bends light beams so that they appear to come from different directions than the original quasar. (Credit astrosociety.org)

Many observations showed galaxies getting lensed due to the gravity of objects that can’t be seen. Many postulated that Dark Matter caused these lensings. Gravitational Lensing caused by many galaxy clusters too advocates the existence of Dark Matter.

Astronomers are quite sure about the existence of Dark Matter that they have included it in their theories too. It appears that a universe as we know it could not exist without Dark Matter in it. Astronomers think that Dark Matter provided the necessary gravitational tug that initiated the creation of the very first stars and galaxies. There isn’t enough ordinary matter to create stars and galaxies. Dark Matter can be imagined as a catalyst for these processes — countless computer simulations about the evolution of the Universe from The Big Bang to today support this idea. Our understanding of Dark Matter suggests that Dark Matter is present around galaxies and in galaxy clusters. Many observations of the heavens back this notion.

The reasons, as mentioned earlier, have convinced almost every astronomer and cosmologist that Dark Matter exists.

What do we know about Dark Matter?

Short answer: Not much!

It would be better to study Dark Matter through the lens of our understanding of Normal Matter seeing that we have studied Normal Matter since the 19th century if not any more early. So let’s get on the same page with our understanding of the “Normal Matter”. Particle physicists believe that the 5% that makes us is made up of 12 fundamental particles and 4 fundamental forces. This is called the Standard Model of particle physics. I would like you to remember from your school days that there are objects called “Atoms” that make you and everything. Well, those atoms are made up of these 12 particles, and 4 forces govern them. Let’s look at the four forces.

Gravitation force: That thing that comes from apples and given by Issac Newton, my man! This force causes the attraction between matter particles. This is the weakest force.

Electromagnetism: The force responsible for the attraction and repulsion of charges and the behaviour of magnets. It is also accountable for the propagation of light.

Weak Nuclear Force: It is similar to electromagnetism but supper weak. If Electromagnetism is on steroids, then the weak force is on anti-steroids. (if that is even a thing!)

Strong Nuclear Force: It is the most potent force, and it is responsible for the stability of the nucleus of the atom. Without it, the protons in the nucleus would fly apart due to electromagnetic force.

It is widely believed that the Dark Matter only interacts with the gravitational force. Hence it must have mass, and therefore it is ‘Matter’. It is ‘Dark’ because it doesn’t interact with the electromagnetic force, responsible for the propagation of light. This is the reason why it is called “Dark Matter “. This is the only property astronomers think certainly exists.

The particle nature of Dark Matter is not very well understood. We are not quite sure whether it’s even made up of particles as the ordinary matter does. We must remember that just because one kind of matter is made up of particles that don’t imply that every kind will show this property. Another way to look at this is that just because you had pizza for dinner today doesn’t mean that all dinners are pizza! This is crucial in science. One needs to keep an open mind when tackling the mysteries that vex us. We must consider every possibility and try to test any predictions that they might suggest by conducting experiments. One must then cancel out any false possibilities by sceptically analysing the results of the experiments performed. So Dark Matter may be made up of giant Minions floating in space but testing that would be very challenging.

So to keep matters simple, physicists hypothesised small particles that don’t interact much and have a lot of mass to it. Those particles are called WIMPs short for Weakly Interacting Massive Particles. In common words, something that has a lot of mass and doesn’t interact much. (something just like me) It is believed that WIMPs interact with our kind of matter very very weakly with a new hypothesised force. This force is out of the previously known forces. Once astronomers believed that less bright objects like failed stars, black holes, neutron stars, primordial black holes, and rogue planets ( planets without a star) constituted for Dark Matter. These objects were called MACHOs for Massive Astrophysical Compact Halo Objects. Boy! Astronomers like to name stuff, don’t they? But, studying the heavens have revealed that MACHOs are not quite enough to constitute 27% of the universe.

Other candidates include particles like Axions and Sterile Neutrinos. However, as of now, no experiments have detected any of these particles. Many particle physicists and astrophysicists would love to see either Axions or Sterile Neutrinos to make up the Dark Matter as they would solve not just Dark Matter but other mysteries of the quantum universe. If the situation was not weird enough that some physicists suggest that the gravitational force might behave differently on larger scales. They have postulated revised theories of gravity. However, these theories fail to explain our universe any better than our previous understanding.

The most surprising behaviour of Dark Matter is that it appears that it may not even interact with itself! The observation of the collision of two galaxy clusters suggested the above notion. When the galaxy clusters with Normal Matter and Dark Matter collided, all the ordinary matter collided and behaved as normal matter normally does. But, what happened when the Dark Matter crashed ?, Nothing! (Nothing that we could detect happened. there may be other means of their interactions).

It appeared that the Dark Matter present in both the clusters seemed to be ‘Unaffected’ as the clusters collided! This means that when Dark Matter collided, it passed straight through each other unaffected. That is creepy and awesome at the same time.

So Dark Matter doesn’t interact with normal matter and other Dark Matter. Gees! You must be wishing that you were Dark Matter now! Imagine if you were, you could just pass-through traffic singing to Chromatica! No? Maybe that is just me!

How can we study Dark Matter?

Trying to study and understand something that can’t be observed is no easy task. We humans who can’t even navigate ourselves in the dark without bumping into something are trying to study something invisible. Wonderful! Getting hands-on some Dark Matter will help us understand this enigma. Many experiments are designed and are being conducted to detect Dark Matter particles. For instance, experiments are being undertaken underground to detect any dark matter particle’s interaction with noble elements like Argon, and Xenon. To increase the sensitivity of the detector these experiments are being conducted underground. The ground above the detector acts as a filter and blocks any other particle that might mimic WIMPs. These experiments are designed hoping that Dark Matter will interact with ordinary matter. Without the assumption that Dark Matter can interact with normal matter, the situation gets more complicated.

The LUX-ZEPLIN dark matter experiment will be located one mile underground at the Sanford Underground Research Facility in South Dakota, in a cavern within the former Homestake gold mine. (Credit https://news.wisc.edu/)

There is an exquisite piece of engineering under the Swiz-French countryside that accelerates beams of protons close to the speed of light and studies the aftermath of their collisions. This beauty is called the LHC short for Large Hadron Collider. This machine harnesses the true beauty of the world-famous equation “E = mc²”. In English, this equation means that energy and mass are the same things.

The energy of the collisions is used to create new particles. This machine has refined our understanding of the quantum cosmos. Here physicists hope to create Dark Matter particles from the energy of the collisions. One plus side to these experiments is that we might detect more particles which will refine our understanding of other fields of study in the particle universe. As of today, no such particles have been detected. But, more high energy collisions might create these particles one day. Conducting such high energy collisions requires a lot of energy which is very feasible right!

This is a birds-eye view of LHC and other experiments. (Credit https://www.tagesspiegel.de/)

The universe itself is no short of a high energy particle collider. There are high energy astronomical processes that accelerate particles at very high speeds and can collide them too! In these processes, regular matter collisions can make Dark Matter and similarly Dark Matter collisions can create ordinary matter.

So we can look for these processes and analyse the aftermath to detect any increase in the density of ordinary matter or dark matter. This is hoping that Dark Matter interacts with the ordinary matter with some process!

Detection of any Dark Matter particle will revolutionise our understanding of the universe overnight! I can’t wait for that to happen!

This is a Quasar. This is what happens when a supermassive black hole at the centre of a galaxy starts to eat. It is one of the most extreme locations in the Universe so a perfect candidate for searching Dark Matter. (credit makeagif.com)

Why does Dark matter even Matter?

One finds a hard time resisting this, as here we are trying to detect something that doesn’t even interact with us and is hardly a concern for any government. All we know about the dark matter right now is that it exists, it has mass, its invisible, and it may* not interact with itself. The common man will hardly be bothered if there are particles like WIMPs, Axions and Sterile Neutrinos. But, consider the following. No one could have imagined the outcomes of J.J Thompson’s discovery of the Electron or the scattering of Alpha Particles by Rutherford. The electronic device you are using to read this is working on the aftermath of the quantum revolution.

So currently we may not have any idea what dark matter is but think of the possible outcomes of this endeavour. In our quest to study the invisible, we may very well discover a more fundamental nature of the cosmos. We might discover elements made up of dark matter! Something like Dark Hydrogen, Dark Oxygen or Dark Carbon. We might have dark physics, dark electronics, dark chemistry (although chemistry is already dark!), dark music, dark philosophy! ( that is a terrible idea). We might learn new ways to manipulate ordinary matter. Just think of the possibilities here.

Apart from the above fantastic possibilities, the most compelling reason to study the unknown is the joy of discovery. The joy of discovery helps us know a little more about the universe and our place in it. The cosmos is enormous and complex. No one understands it for sure, and that’s why we do science. We can’t say that we fully understand the cosmos when we have no clue about the 27% of the stuff in it! So let’s continue studying Dark Matter. Who knows one day we might discover a process that transforms normal matter into Dark Matter so that we could just pass-through traffic singing to Chromatica!

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