Dark Matter: The Unknown Force
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What if I told you that your entire life, everything you've ever seen, everyone you've ever met, every cluster of galaxies, stars, our planet, only makes up for less than 5% of the entire universe? The other 95 plus percent is made of stuff that we physically cannot observe; matter and energy which we do not comprehend. This has led scientists onto one profound quest of trying to improve our understanding of what all these mystical universal ingredients might be.
According to Sir Isaac Newton, bodies have a mutual force on each other that is determined by their masses. The greater the mass, the greater the force. In addition, a large body's force on smaller orbiting bodies greatly decreases over distance. During the 1930s, the Swiss American astrophysicist Fritz Zwicky investigated the movements of galaxies within a huge galaxy cluster named Coma, about 300 million light-years away from Earth. Coma consists of thousands of galaxies orbiting around the center of the cluster in all possible directions, much like bees swarming around a beehive.
By studying the movements of a few dozen galaxies to study the gravitational field that holds the cluster together, Zwicky was able to conclude something that didn't make much sense. The galaxy cluster rotated so rapidly that the gravity from the stars and gases and everything in the cluster could not possibly hold itself together. It wasn't even close, actually. The value he obtained said there needed to be at least 400 times the amount of matter in the cluster just in order to keep things held together. The galaxies rotated faster than their escape velocity—the speed that causes a celestial body to leave its orbit. For example, if we could increase Earth's orbital speed by just 1.5 times its current speed, our planet would be thrown out of our orbit around the Sun and leave the solar system for good.
Based on the insanely fast speeds Zwicky observed, the cluster should dissolve quickly and leave only traces of its former existence. But today, the cluster is more than ten billion years old. The universe itself is only 13.8 billion years old. Therefore, Zwicky concluded that there must be much more gravity in the Coma Cluster than what one could see—some sort of matter that we could not see. He went on and gave this missing matter a name. For the first time, the term dark matter was used, and thus the most prolonged unresolved mystery in modern astrophysics was born.
However, Fritz's work was unpolished. The term dark matter Zwicky was using didn't talk about the same type of dark matter that we know today. He knew there was something there; he just didn't know what. In the 1970s, American astronomer Vera Rubin began studying individual stars in individual galaxies. Think of our solar system: because they're closer to the Sun, which by the way accounts for over 99% of the mass in our solar system, planets like Mercury have a much shorter orbital period than planets like Saturn and Jupiter.
Now, if we slice this up to a galactic scale, you would imagine that as you go further towards the edge of the galaxy, the stars at the edge would be moving slower than the ones near the center. Vera Rubin was studying this exact thing, except she discovered the exact opposite. Within each galaxy's normal matter, like stars and planets that are further away from the center of the galaxy, move at speeds greater than the ones near the center. This doesn't make much sense.
As you got further and further from the galactic center, the speeds at which stars moved flattened out when, in fact, they should have decreased. If they moved too fast, they'd be flung out of the galaxy and lost forever. Too slow, and they'd literally fall out of their orbits. But instead, the speed stayed relatively the same. This shouldn't happen. From this discovery, the rabbit-hole known as dark matter gained popularity across the world.
Dark matter is the supposed kind of material that doesn't emit, reflect, or interact with any kind of electromagnetic radiation: things such as light, x-rays, gamma rays, radio waves, and so on. The only way we've been able to figure out that dark matter might exist is through gravity. Gravity is the bandwidth; it's the link between normal matter and dark matter. This sort of introverted matter is the reason why it's nearly impossible to detect.
More direct evidence of the strange nature of dark matter lies in the amounts of hydrogen and helium there are in the universe. The nuclear fusion in the first few minutes after the Big Bang left round numbers like 10 hydrogen nuclei for each one helium nucleus. Calculations showed that if most of the dark matter had participated in nuclear fusion, there would be much more helium in relation to hydrogen in the universe. From this, we can conclude that most of the dark matter—that is, most of the mass in the universe—does not participate in any fusion processes. This disqualifies it as ordinary matter, which very meaning lies in its willingness to interact with the atomic and nuclear power that builds up you and I.
You might be able to say that dark matter and fusion processes just don't go together. According to Einstein, heavy objects can alter the geometry of space-time. Instead of seeing gravity as a result of mass attracting mass, as Newton did, Einstein suggested that space and time bend around objects with mass. Imagine the universe as a flat grid, and objects with mass create a well of sorts in this grid. Objects with more mass, like stars and black holes, create a larger well that has a larger effect on matter around it. Everything follows the curvature of space-time, light included.
Well, it's something else that also bends light—lenses. If we were to imagine a cluster of galaxies, its gravitational pull literally would bend light around itself due to the sheer mass of the galaxy cluster; it creates a huge gravitational well. We could figure out the mass of the galaxy cluster based on how much light from the galaxy is being bent. This is called gravitational lensing.
Scientists use this trick to determine the mass of two colliding galaxy clusters called the Bullet Cluster, about 3.5 billion light years away. When clusters of galaxies collide into each other, they continue to travel. There isn't really even much of a crash. When the Milky Way collides with Andromeda in about four billion years, our solar system should remain unscathed. The sheer amount of literal space between stars is so massive that very few stars or planets will collide with one another. However, the gas in between the galaxies collides and is visibly noticeable.
This is what happened here. The intergalactic gas inside the clusters collided with each other; they heated up and emitted x-rays that we can observe. However, when we use the knowledge of physics as we know it—using Einstein's general relativity and taking gravitational lensing into account—the location of the mass wasn't where we expected it to be. You would imagine that when galaxies collide, the resulting galaxies from the collision would be larger and more massive, and you're right to think this way.
However, when the two galaxy clusters collided, the majority of the mass was actually located on the outside of the galaxies, not inside. This directly aligns with our idea of how dark matter interacts with normal matter. The gases in the collision were slowed down by a drag force from the gases colliding with one another. This acts like something similar to air resistance here on Earth. However, the dark matter wasn't slowed down at all—remember, dark matter only interacts with gravity. Normal matter can collide and fuse and heat up, but dark matter doesn't care.
During the collision, the dark matter in each cluster just continued moving ahead while the gases collided and heated up. This is what you're seeing right now. Scientists studying this claim that the chances of this being a fluke and purely coincidence are one in one quadrillion. As we know, only about 5% of the matter in the universe is normal visible matter. Dark matter accounts for 25%—about five times as much! Something we can't see has a bigger effect on gravity than everything else we can't see because there's so much more of it.
Dark matter essentially lays the framework for the largest structures in the galaxy 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 dense regions of space. This could be an explanation as to why we see so much of it in galaxies. If dark matter is axions, as many believe, they would most likely go right through each other without crashing at all; that's the very nature of axions.
And that's what we observed here. Axions are hypothetical particles and some scientists' biggest bet when it comes to explaining what dark matter is. They weigh roughly a trillionth of the mass of a neutrino, the smallest particles that we're aware of in our current standard model of physics. If somehow axions are the explanation for dark matter, there has to be a lot of them. Remember, if there's five times as much dark matter as there is ordinary matter, and axions are considered to be even smaller than the smallest things we know today, then there must be an absurd amount of them in the universe.
You'd imagine that because there's theoretically so many of them, that it'd be easy to detect, right? No, not really. It's similar to the signal you've received from someone calling your phone from Mars. I think it's easier to explain axions as a wave instead of a particle, and here's why: light itself behaves both as a particle and a wave. You might have heard this before.
As you find particles with smaller and smaller masses, you reach particles that don't have a further particle to decay into. I find it much easier to picture them as waves that permeate throughout the universe—a field of sorts. When we detected gravitational waves in 2015, we used detectors that were 16 square kilometers in order to detect a motion that was ten thousand times smaller than the nucleus of an atom. However, when studying axions, the experiments could essentially take place in your house if you wanted.
The Axion Dark Matter Experiment, or ADMX, is essentially just one big magnet. If there are axions around, this massive magnet literally converts these dark matter axions into microwave photons—light, which we can luckily see. ADMX is one of the only detectors in the world capable of detecting axions. A similar experiment took place in 2016; students at MIT created another experiment to try and detect axions. It went under the name “A Broadband / President Approached a Cosmic Axion Detection with an Amplifying B Field Ring Apparatus,” or ABRACADABRA. Funny, however, so far we haven't had any luck with either of these experiments.
Nevertheless, we do not know what dark matter really is. Some believe that dark matter does not consist of any matter at all—not normal, not dark, not anything else—but of something completely different. Something that has mass but doesn't behave like anything we know. Perhaps one day we find out that dark matter does undergo fusion processes—a dark fusion of sorts. If we could harness that energy, who knows what things we could use it for?
Reaching into the realm of science fiction, just maybe we can see the effects of forces from other dimensions, other universes. Perhaps the usual gravity from ordinary matter crosses a thin film from a phantom universe adjacent to our own. Maybe that could be the explanation for dark matter. If so, if we can feel forces from external universes, our entire universe can only be one of an infinite set that together formed the multiverse.
But let's not get too crazy. After the Big Bang, the universe at that time was full of an atomic soup. It was hot and dense but rapidly expanding. Dark matter and dark energy have been in a battle against each other since the Big Bang. Today, as we know, dark energy is currently the dominating force in the universe, but in the early ages, when the universe was much younger, hotter, and denser, dark matter was actually the stronger of the two. Believe it or not, we owe our entire existence to dark matter.
If you took the universe as we know it and kept everything except to remove the dark matter, we wouldn't be here today. Galaxies wouldn't have been able to form in the way they did without the assistance of dark matter. Normal matter wouldn't have come together the way they did to form the Milky Way we know today. Dark matter has allowed countless galaxies to form with billions of stars and planets; they're all structured in relatively the same way, more or less, thanks to dark matter.
At least one planet was able to exist in the right conditions for evolution to take place for us to be here today. Discovering what dark matter truly is, it's just as important as Einstein's theories of relativity. It's just as important as discovering the expansion of the universe. It's a monumental step to the next levels of mankind. The quest for dark matter is a journey, as is everything else important in life. Sure, the end goal is great, but the discoveries and experiences you make along the way are what makes the entire process worth it.
For example, because of the space race in the late 1900s, we have things such as laptops, GPS, camera phones, and plenty of other things that we take for granted each day. We have all of these things because of the nature of humanity. We're competitive and territorial creatures by default, but because of this, we're pressured in our own ways to better not only our own lives but also the lives of those around us.
Well, as time goes on, with every discovery we make, we realize that we're becoming more and more insignificant in the grand scheme of things. However, these discoveries are the only way for us to push mankind further than any other species has. It's like a two steps forward, one step back relationship. Something that thousands of physicists across the world have been searching for is in the same room as you are right now. The things that appear to be the most common things in the universe are actually some of the rarest.
So, whenever you look up at the sky and wonder what lies beyond our reach, just remember: you can't see everything.
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