Today's episode will be a bit trickier, this is why I’m gonna start with a disclaimer. First of all, I am not an astrophysicist, I’m just a science enthusiast, and I’d have loved to have, for this episode, the contribution of an astrophysicist. So I’m gonna try to explain this as clearly as I can, considering that this topic is something that more than one astrophysicist want to understand. So, knowledge on this topic evolves constantly and of course, one day, I hope, we will discover the true nature of what is dark matter and dark energy. So this episode is written in November 2023, at the best of my knowledge, but what I’m saying today could become inaccurate or inexact very soon in light of future discoveries.

So, what is dark matter? Well, let’s imagine that I give you a balloon, that has stuff hidden inside. This balloon can be infinitely extended. I also give you a bottle of helium. You take the balloon, and inflate it with the helium inside, and what’s gonna happen: well, the balloon will inflate. Dark matter is the helium inside the balloon. Okay, but I also talked about dark energy, what is it: well, now imagine that, after you inflated your balloon, it keeps inflating, and you have no idea why. That is dark energy. The problem in all that, it’s that, just like in your balloon, dark matter accounts for nearly over eighty percent of the entire mass of the universe. But what is it? Well, we have no idea. And do we know if this stuff is at least real? Turns out that, yes. On that, we are quite sure, actually.

So, in order for you guys to understand what we’re talking about today, I’ll speak in example and metaphors. As the name suggest, no, guys, I won’t explain you guys what dark matter, or dark energy is, because, this is something that no one really truly understand now. What’s quite paradoxical about dark matter is that, we know it’s there. We don’t even know if we should talk about matter, in fact. We calculated it, and we know that, if it doesn’t appear in our equations, then the result doesn’t make any sense. But if we have no idea as to what this is, we, however, have an idea as to what it is not. And today, we’re gonna go through that together.

Now, for you guys to understand everything, let’s talk about gravity. Because, yeah, what is gravity? You may have cursed it when something fell on you, like this glass of wine accidentally dropping on your dress or suit, but, what is it? Well, someone named Isaac Newton in England in the seventeenth century asked himself the same question. He allegedly saw an apple falling off a tree but, this is apparently a legend. But either way, something clicked in his mind, and he gave us the famous Newton Laws, explaining gravity. You certainly have seen it in school. It’s on how to calculate the mass of an object that we can determine at what speed it will fall. But, that was the problem: he based himself on Johannes Kepler, a German astronaut, who understood by this time that planets were turning around the sun… so controversial laws back in the days, due to the strong influence of the church, but Newton finally explained that, what could attract planets to turn around the sun could in fact be the same thing that makes an apple falling from the tree. And that discovery was indeed important, still considered as a major scientific discovery and advancement of the century.

But, like I said, Newton explained how it worked. But didn’t explain why it worked.

Someone explained why it worked in Switzerland, in 1900. From the comfort of his office, he was probably bored that day, and, he looked at his window, at the Patent Approval Office of Switzerland. From this morning, someone was painting the wall, and this guy was on a ladder. Lost in his thoughts, in some sort of strange questioning, he started thinking: let’s imagine that this guy falls from his ladder. What would fall first, in terms of time: is it him, or is it the pencil he carried or the painting pot next to him? Then it all clicked, because during his fall, he will travel through space, so the top of his ladder to the floor, and time, because it will take him a certain time to fall. And in the end, nothing would fall first, everything would fall at the same time and impact the ground at the moment it started falling.

And when we say that civil servants are lazy and on-the-clock employees, well. This guy would prove you wrong.

This guy was Albert Einstein and times after this deep thinking afternoon he penned the first draft of what would become a major discovery: the Theory of General Relativity. General relativity explains that gravity is in fact linked with space in time, and it’s all about that in the end. How it works, its pretty simple: let’s imagine that Earth is a ball and this ball is on a carpet. This carpet is the space. Now, let’s imagine that you make the ball starting spinning on itself, at a certain speed. What’s gonna happen, is that, over time, the carpet, near the ball, will slightly twist, all around this ball. And that twist is in fact the gravity generated by the ball when it started spinning on itself. Pretty simple, right?

Einstein’s publication was a massive breakthrough in science back in the days, and even today, and paved the way of a clearer understanding of how the world worked and led us to create fantastic new things, and more importantly, explained everything. And since we knew it was all about time and that space and time are both very closely linked, we also understood that nothing, absolutely nothing, can travel faster than light in space. This is explained by the equation you guys have surely seen somewhere, the famous E=MC2, which is the calculation of, how much mass will it require in order to travel faster than light. And why “faster than light”? Well, what we call light is in science called a photon. And, a photon is a small particle, or at least if I could call this like that, that had no mass. Yeah, there’s nothing smaller than a photon of light. So what object with mass could go faster than something with no mass? Well, nothing. At least for now.

So now, perhaps one of those summer days, you were outside, at least out of a big city, and, lying down in the grass, you looked during the night at the stars and said, well, it’s far away… and you probably picked your interest as to how far this star was compared to you. And you found that this star was at a certain light years away from Earth. I know, I’m doing that with my wife when we’re in France, because we’ve got a clear sky near the city I am from, and it’s magic to be lost in the immensity of space. And you were probably dazzled by the fact that thy star was, let’s say, 10 millions light years away from you. But you were like, wow, it’s far… but probably wondered what a light year away means.

Then you told yourself that a light year is the distance this light, so the photon we’ve been talking about, travelled through space to come to your eye and be absorbed. And you were right. But it’s here that things make sense: during those ten millions light years, at least, in the photon’s point of view, well… no time happened, since this photon was travelling at the speed of light, but in space. So what that means, is that from the moment it was emitted to the moment it was absorbed, technically, no time happened for it. It was not even born that it was already dead. But from your point of view… because you’re also travelling at the speed of light, but in time, (because, unfortunately, you cannot go faster than the time that scrolls before your eyes) then you were the lucky one to actually be there to absorb this photon in your eye and see that star shining.

Now… it’s kind of poetic to say that, but in summary, when you look at the sky in the night, well, you’re in fact looking at the past. Yeah. Think about that next time you’re looking at a star in your garden.

Now, let’s talk about something you all experienced once in your life. It’s called the Doppler Effect, discovered by a Danish scientist, Christian Doppler. What is it? It’s precisely that.

So, what that means in here, it’s that, when an object move in space, it compresses the waves before it and, as a result, extend the one that trails it. In a nutshell, when the siren you heard was coming close to you, its sound seemed more low-pitched, or seemed like a normal siren. However, once it passed after you, the sound, as it was compressed in front of it, left a high-pitch trail, and… this was why you heard it in a lower pitch. But of course, the Doppler effect does not only apply to sound. We discovered it through the sound, indeed, because that’s the only observable way we had when we discovered it, but it does also apply to light.

Again, we are back in space and time. See, Science is amazing. Everything is connected.

Now, if you aren’t already sleeping by what I just said, you paid attention to when I said “when you look at the sky in a clear night, you look in the past”, you know, stars, those little white or yellow dots forming drawings in the sky… yeah. Well, you see them yellow, or white, or whatever. Yeah, but, guess what? They aren’t white at all. Nah. They are in fact either blue or red. You see them white because the Earth atmosphere makes you see them white, but the reality is different. And that, it’s someone else, Edwin Hubble who explained it. Hubble, if you know a bit about astronomy, it’s the guy who gave his name to a famous telescope that brought us fantastic images of the Cosmos, but Edwin Hubble was also another scientist who made major discoveries on our understanding of the universe. And, amongst the plethora of stuff he discovered, he actually determined that when you see a star with a red glow through a telescope, it actually means that this star comes to you. Well, not to you personally, but it comes close to us, on Earth. On the contrary, if a star has a blue glow, well, it means that it moves far away from you. But of course, since we talk about things moving in the universe, we are talking about huge, tremendous speeds.

So now you guys are gonna tell me, okay Taylor, fine, but what does all that has to do with Dark Matter?

You remember when I said that, thanks to the General Relativity Theory, we could start measure the mass of object, galaxies, and other stuff? Back in Switzerland (I know, there again), in 1933, an astronomer named Fritz Zwicky took samples of galaxies located in the Coma Berenices constellations, and started doing the maths. Don’t ask me how he did, but his idea was, how do we calculate the mass of an entire galaxy, so we could determine how much weight our galaxy, the Milky Way. But Zwicky had, erm… let’s say… a certain attitude towards his colleagues, and this thing made that, well, let’s say, he wasn’t that much in good terms with lots of his colleagues. But the thing was, once he did the maths, something didn’t add up right at all: the values he found as a result of his equations were a hundred times more than what he predicted.

A pretty bad variance, indeed.

But like I said, Zwicky was not liked by lots of his colleagues, so his work became, in the end, quickly dismissed by his colleagues, and everybody argued that, because of errors of calculations, he could never have found the correct result… but, this was not calculated either. And no one really did the maths to get the result. You don’t want to help a colleague that you hate, don’t you? But what if he was actually right when he did the math?

No, instead of that, in 1960, technologies evolved, and we could now use the radio, and we started using radio telescopes to observe and take pictures of the universe. In fact, the Roswell affair wasn’t very long ago, and as everybody was looking for alien life whilst the US government was spying on the USSR, in the middle of the Cold War, a coalition of scientist launched the SETI project, SETI standing for Search for ExtraTerrestrial Intelligence. And, you get the idea: it was all about listening to the sky and search for aliens. But not only: as their started pointing their telescopes in the sky, apart from a signal that will remain famous as the Wow signal, that I will definitely talk about in a further episode, they actually observed something that didn’t make any sense at all: on the outer regions of galaxies, stars were spinning at an equal speed as they were spinning in the centre of their galaxies. And… that did absolutely not make any sense to them.

So now, do you remember why I explained gravity at the beginning? Here we go: Newton and Einstein taught us that, when a body starts turning on itself, it wraps the space-time and it creates gravity, and at the same time, Doppler and Hubble tell us that when an object glides towards us, or away from us, it emits a red or a blue light. So now, we faced another problem. Because if the universe was stable and static… then why did stars on the periphery of galaxies would turn at the same speed than those close to the centre of the galaxy, right? It doesn’t really make any sense, does it?

So, coincidence? No, absolutely not, and this will pave the way of something that will become one of the greatest scientific mysteries ever: if all the stars within a galaxy are all turning at the same space, whether they were close or in the edge of the galactic centre… that was the evidence that the universe was expanding. It wouldn’t make sense otherwise. And now, everything made sense: Zwicky’s calculations, the observations… there was something. So we did the maths again. And again. And again. And they tried removing that mysterious thing from the equation. Because, after all, it’s probably just a coincidence. And turns out that, removing that thing… did not make any sense and made the calculation wrong. If we added it… then everything matched.

And what’s that thing? Astrophysicists call it Dark Matter, and Dark Energy. And why dark? Because we have absolutely no idea of what this is. And we aren’t even sure if we should talk about matter and energy.

Now, let’s talk about the basics: dark energy and dark matter are not the same thing. Dark matter is something that we know accounts for about eighty percent of the mass of galaxies, ours included. Dark energy, it’s the energy that makes the universe inflating. And we know that it’s here. We know that universe expands, through calculations and observation, and we know that something is expanding the universe. But what causes it? We don’t know. Is dark matter and dark energy related? It is apparently unlikely, but not excluded. At this hour, what these are, we strictly don’t know. The only thing we know is that they are here, and without these, we cannot calculate the mass of galaxies and understand why the universe is inflating. But, we may not know what it is… but we know, however, with certainty, what it is not.

But let’s go first on what we know about this dark matter. First and foremost, it may does make sense but dark matter is of course not ordinary matter, since it is not made up of the atoms and molecules that form the ordinary matter we encounter in everyday life, like gases, dust, planets, stars, or living organisms. As I said before, we don’t even know if we should call it matter. It does not emit or reflect light, unlike stars and planets, which absorb, or reflect light, or any other form of electromagnetic radiation. Which is why it cannot be seen directly using optical telescopes. It was not detected directly: Despite extensive research, dark matter has not been directly observed. Its presence and properties are inferred from its gravitational effects on visible matter, radiation, and the large-scale structure of the universe. We also know that it is not uniformly distributed across the universe. It forms a complex cosmic web, with dense regions forming the gravitational ‘skeleton’ that galaxies and galaxy clusters appear to be built upon. It is not the same as Black Holes: as they are also invisible and detected by their gravitational effects, they are objects of extreme density formed from collapsed stars or other processes. Dark matter is thought to be more diffusely distributed throughout the universe. We also know that the current standard model of particle physics, which explains fundamental forces and particles, does not include a particle that could account for dark matter. This suggests that our understanding of physics might need to be extended. And finally, it does not not explain all cosmic phenomena: even though it is crucial for explaining the rotation rates of galaxies and the motion of galaxy clusters, it does not explain other phenomena, such as the nature of dark energy or the specifics of black hole formation.

Now, regarding what we know about the little brother, first, we know that dark energy is not a dark force: Unlike what the name might suggest, dark energy is not a force in the traditional sense like gravity or electromagnetism. It doesn’t push or pull objects in a directional manner. It is not Dark Matter either, because while dark matter contributes to the mass of the universe and affects the motion of galaxies and clusters through gravity, dark energy is thought to drive the expansion of the universe. It is not a known energy form, as Dark energy is not a form of energy that we encounter in everyday life or in known physics. It’s not like electricity, nuclear energy, or chemical energy. It is not visible or detectable directly: we can’t see or directly detect dark energy with current technology. Its existence and properties are inferred from its effects on the universe’s expansion and cosmic structures. Perhaps we shall in the future. It is not static or unchanging: The understanding of dark energy could evolve with new observations and theoretical developments. It’s not a static concept in cosmology. It is not necessarily constant either: while the simplest model of dark energy is the cosmological constant, which implies that dark energy has a constant density throughout space and time, this is not definitively established. Its properties could vary across time and space. And in the end, it is not the solution of all cosmic mysteries, because while dark energy helps explain the accelerated expansion of the universe, it doesn’t answer all the unknowns in cosmology, such as the nature of dark matter or why the Big Bang occurred.

But we know it’s there. We can calculate it, and we can see its effects, so, from a mathematical point of view, it’s impossible to ignore. So, we started making hypotheses as to what it could be.

It could be what we call WIMPs, for example. Okay, so, said like this, it’s a bit rude indeed, but a wimp is in fact the acronym of Weakly Interacting Massive Particles. These hypothetical particles interact with gravity and possibly weakly with other particles, but not with electromagnetic force, making them invisible to light-based observing technologies. But there is not enough evidences to sustain this. Amongst the candidates are also the Axions. It’s another theoretical particle, and they’ve got a lot in common: axions are extremely light, much lighter than neutrinos. They are hypothesized to solve certain problems in particle physics and could be a component of dark matter. Now, what’s an axion, you’re gonna ask me? Well, think of another super tiny particle. It's pretty much like a secret agent particle that nobody has ever seen. Scientists think axions could be floating all around us, but they are very good at hiding. They're so good at it because they're incredibly light and barely ever touch anything else. Like dark matter, axions are really hard to find, so they could be a form of it. But, again, no evidence.

And as we talked about Axions, as I mentioned its name before, we’ve got another suspect, called the neutrinos. What is it, well, it’s pretty simple. Imagine a tiny, tiny particle, that is so small and light that it can zip through walls, mountains, and even the entire Earth without stopping or even touching anything! I know, physics can be really funny sometimes. Neutrinos come from the sun or other stars from space and are always moving around us, but we can't see them or feel them because they're very shy and hardly ever interact to other particles. But whilst it has been widely eliminated that dark matter could be neutrinos, it remains that it could be a specific category of sterilised neutrinos, like a type that interacts only via gravity, making them again another potential candidate for dark matter. Unlike regular neutrinos, they do not interact via the weak nuclear force. So there is still potential there.

It could also be something called MACHOs, another acronym for Massive Compact Halo Objects. Yeah, I know, scientists love funny acronyms. Between Wimps and machos, we’ve got for all colours indeed. Anyway, what’s a MACHO in science? In a nutshell a plethora of stuff that include objects like black holes, neutron stars, and brown dwarfs. They are massive enough to exert gravitational effects but do not emit light. However, extensive searches have provided limited evidence for MACHOs. So, back to zero again here. But since we talked about this, it could also be Primordial Black Holes: These hypothetical black holes formed soon after the Big Bang and could account for dark matter. Unlike black holes formed from stars, they could be of various sizes, including very small ones. So we would be surrounded by black holes in this theory, and… again, no evidence.

Or perhaps understanding dark matter could in fact change our perception of the world we live in and, perhaps, update the current gravity theories we already established: Some theories propose modifications to our understanding of gravity, which could explain the gravitational effects attributed to dark matter without invoking unseen matter. This could make sense since, when Einstein explained General relativity in 1905, he also predicted the existence of black holes, that we never thought would ever exist. The latest theory predicted by Einstein and that got verified was the gravitational waves, so perhaps, in light of future discoveries, we will have the key to decipher the enigma. Or perhaps, in Einstein’s theories, there is something big that we have not seen?

Many other theories have been evoked, such as a hypothetical Superfluid Dark Matter, a theory that combines aspects of particle physics and condensed matter physics, suggesting dark matter could have superfluid properties under certain conditions, influencing galaxies’ rotation. Or Self-Interacting Dark Matter, a model that proposes that dark matter particles interact with each other through forces other than gravity, which might explain some observations of galaxies that don’t align well with the traditional dark matter model. Or Composite Dark Matter, where dark matter is made of composite particles, similar to how protons and neutrons are made of quarks. Or possibly Dark Photons, Analogous to photons in electromagnetism, dark photons would be the force carriers for dark matter, interacting with it in a way similar to how light interacts with ordinary matter. But for now, all we know is that dark matter surrounds us. It’s everywhere, and something we can’t ignore. It accounts for nearly 80 percent of the mass of the galaxy. And this eighty percent remain completely oblivious to us.

Now, to conclude this episode, I would like to bring up something for you guys to meditate on. When scientific discoveries are made, a scientist, in order to remain credible when they will be asked about their discoveries, will always try to prove the fact that their discovery is in fact wrong. Why they do that, it’s actually to make sure that, what they discovered is in fact true, because it can’t be proven otherwise. It takes years sometimes to find something accurate, something that does make sense, and in the end, when this is confirmed, it’s the moment for the scientist to publish their discovery and share it with the community and the world. Now, I’m gonna ask you something: do never say that you are sure of something until you completely are. As Dark Matter and Dark Energy serves as the perfect example, always say that, it could be, unless proven otherwise. No one can be sure at the moment to have found what dark matter is… unless this person will be published in Nature very soon and will receive a Nobel Prize. Always gather evidence of what you’re saying. Because, it’s never funny to be proven wrong.

As we wrap up this journey through the shadowy realms of the universe, discussing the enigmatic dark matter, we are humbly reminded of the vastness of the unknown that lies beyond our current understanding. Dark matter, an unseen force holding galaxies together, yet eluding direct detection, challenges our perceptions of the cosmos. In the grand scheme of things, it represents the immense mystery that is our universe, an ever-present reminder of how much is still left to discover. As you guys reflect over this cosmic mystery, I invite you to share your thoughts in the comment section below. What might the unraveling of dark matter’s mysteries tell us about the fundamental nature of reality? Until our next rendezvous with the enigmatic and the inexplicable, keep your minds curious and your eyes on the stars. Farewell, fellow seekers of the unknown.

To Matthew Perry