All the clusters of galaxies that we can see only make up for less than 5% of the universe. All the remaining 95+% is made of stuff that cannot be observed. This has led scientists on a quest to improve our understanding of these mystical ingredients.
According to Sir Issac Newton, bodies have a mutual force on each other that depends on their masses. The greater the mass, the greater the force. In addition, a large body’s force on the small body significantly decreases with distance. During the 1930s, the Astrophysicist, Fritz Zwicky, investigated the movements of galaxies in a cluster called Coma about 300 light-years away from Earth. By studying the movements of a few dozen galaxies to study the gravitational field that holds the cluster together, Zwicky concluded that it didn’t make much sense. The galaxy cluster rotated so rapidly that the gravity from the stars and the gases could not hold the galaxy together. The value he obtained said that there needed to be 400 times the matter that was in the cluster just to hold everything together. Galaxy rotated faster than their escape velocity, the velocity needed to escape the gravitational pull. Based on the insane speeds the galaxies were moving, the cluster should dissolve quickly and leave only traces of its former existence. But today the cluster is more than ten billion years old. Zwicky concluded that there must be much more gravity in the Coma cluster, some matter that we cannot see. He gave that matter a name, Dark Matter.
In the 1970s, American astronomer Vera Rubin began studying individual stars in individual galaxies. Think of our solar system, because they are closer to the sun, planets like Mercury have much shorter orbital distance than Jupiter. Now, slice this up to a galactic scale. You’d imagine that when you go to the end of the galaxy, the stars will move slower. Rubin was studying this exact same topic. But what she discovered was the exact opposite. Within each galaxy, stars and planets that are further away from the centre of the galaxy move at a speed greater than those near the centre. As you moved further and further from the galactic centre, the speed increased. The general definition of dark matter goes as:
Dark Matter is a hypothetical matter that doesn’t emit, reflect or interact with electromagnetic radiation.
The only way we know that dark matter exists is through gravity. Gravity is the link between normal matter and dark matter. More direct evidence of the strange nature of dark matter lies in the amount of Hydrogen and Helium in the universe. The nuclear fusion in the first few minutes after the big bang left round numbers of nuclei. Like ten hydrogen nuclei needed for one helium nucleus. Calculations showed that if dark matter took part in the fusion reactions, there’d be much more hydrogen and helium in the universe. So it proves that dark matter doesn’t take part in the fusion reactions, making it different from the normal matter.
According to Einstein, heavy objects can alter the geometry of space-time. Einstein suggests that space-time bends around objects with mass. Imagine the universe as a flat grid. Objects with mass create a well in this grid. Everything follows the curvature of space-time, light included. Something else that also bends light is lenses. If we imagined a cluster of galaxies, its gravitational pull would literally bend light around itself due to the sheer mass of the galaxy. It would create a huge gravitational well. We could figure out the mass of the galaxy by the amount of light that is bent. This is called Gravitational Lensing.
Because there’s so much of it, dark matter essentially lays the framework for the largest structures to form. Dark matter follows the gravitational wells that are formed by an extremely dense region of space. Gravity acts as a funnel and dark matter collects together in these regions. This may be the reason why we see so much of it in space. If dark matter is fundamentally axion particles, they would move past one another without colliding. Axions are hypothetical particles and some scientists biggest bet when it comes to explaining dark matter. They weigh roughly a trillionth of a neutrino, the smallest particle that we are aware of in our current knowledge of physics. Remember, if dark matter is five times than any other matter known and axions are smaller than the smallest particle known, there must be an absurd amount of them in the universe. If there are theoretically so many of them, they should be easy to detect, but that’s not the case. It’s easier to describe axions as a wave instead of a particle, similar to how light acts as particles too. When the particles become smaller and smaller, there is no smaller particle that they can decay into. Therefore, it is easier to picture axions as waves that permeate through the universe. When gravitational fields were detected in 2015, detectors that were 16 sqr/km in order were used to detect a motion that was ten thousand times smaller than the nucleus of an atom. The Axion Dark matter experiment or ADMX is essentially just one big magnet. If there are axions around, this massive magnet would convert these axions into microwave photons. ADMX is the only detector capable of detecting axions. A similar experiment to detect axions was designed and executed in 2016 by students at MIT. It was named, “A broadband/president approach to cosmic axion detection with an amplifying B-field ring apparatus” or ABRACADABRA. However, so far neither of these experiments has produced compelling results.
Discovering what dark matter really is, is as important as Einstein’s theory of relativity. Or the knowledge of the expansion of the universe. As time goes on, we realise how insignificant we are in the grand scheme of things. However, these discoveries are the only way to push humanity forward than any other species has. What physicists are trying to find is right there in your room. So whenever you look up in the sky and wonder what lies beyond our reach, just remember, you can not see everything.