We Have No Idea: A Guide to the Unknown Universe

by Jorge Cham

In the pages ahead, we’ll explain what the biggest unanswered questions in the universe are and why they are still mysteries. By the end, you’ll have a deeper grasp of just how absurd it is to think that we have any clue what’s going on or how the universe really works. On the upside, at least you’ll have a clue as to why we don’t have a clue.

That pie looks pretty mysterious. Only 5 percent of it is stuff we know, including stars, planets, and everything on them. A full 27 percent is something we call “dark matter.” The other 68 percent of the universe is something we barely understand at all. Physicists call it “dark energy,” and we think it is causing the universe to expand, but that’s about all we know about it.

Dark matter is everywhere. In fact, you’re probably swimming in it. Its existence was first proposed in the 1920s and first taken seriously in the 1960s when astronomers noticed something odd about how galaxies were spinning and what it meant for how much mass was inside them.

Astronomers at first tried to guess the mass of galaxies by counting the number of stars in them. But when they used this number to compute how fast galaxies should be spinning, something didn’t match up. Measurements showed that the galaxies were spinning faster than was predicted by how many stars they contained. In other words, the stars should be flying off the edges of the galaxies, just like the Ping-Pong balls in the merry-go-round. In order to explain the high rotation speed, astronomers needed to add a huge amount of mass to the galaxies in their calculation so all the stars held together. But they couldn’t see where this mass was. This contradiction could be resolved if you assumed there was a huge amount of some kind of heavy stuff that was invisible, or “dark,” in each galaxy.

Another important clue that convinced scientists that dark matter was real was the observation that it can bend light. This is called gravitational lensing. Astronomers would sometimes look out into the sky and spot something strange. They would see the image of a galaxy coming from one direction. There’s nothing weird about that, but if they moved the telescope a tiny bit, they would see the image of another galaxy that looked very similar to the first galaxy. The shape, the color, and the light that came from these galaxies were so similar, astronomers were sure they were the same galaxy. But how could this be? How could the same galaxy appear twice in the sky? Seeing the same galaxy twice makes perfect sense if there is something heavy (and invisible) sitting between you and this galaxy; this invisible heavy blob can act like a giant lens, bending the light from the galaxy so it appears to be coming from two directions. Imagine that light leaves this galaxy in all directions. Now picture two light particles, known as photons, coming from that galaxy and headed slightly to either side of you. If there is something heavy between you and that galaxy, the gravity from that object will distort the space around it, causing the light particles to curve toward you.

As the two galaxy clusters slammed into each other, the gas and dust from the two clusters collided with spectacular results: big explosions, giant clouds of dust getting ripped apart. It’s a special-effects extravaganza. If it helps, visualize the collision of two huge piles of water balloons tossed at each other at crazy high speed. But astronomers also noticed something else. Close to the collision site, they noticed two giant clusters of dark matter; of course, this dark matter was invisible, but they could spot it indirectly by measuring the distortion the clusters were causing to the light from the galaxies behind them. These two dark matter clusters seemed to be moving along the line of collision as if nothing had happened. What astronomers have pieced together is this: there were two galaxy clusters, each with both regular matter (mostly gas and dust with some stars) and dark matter. When the two clusters collided, most of the gas and dust crashed together in the way you expect normal matter to do. But what happens when dark matter bumps into other dark matter? Nothing that we could detect! The clusters of dark matter kept going and passed through each other—almost as if they were invisible to each other. The stars also mostly passed through, because they were so sparse. Enormous blobs of matter, bigger than many galaxies, passed right through each other. In essence, the collision stripped the gas and dust from these galaxies.

Another approach is to try to create dark matter using a high-energy particle collider, which boosts normal matter particles (protons or electrons) to crazy high speeds and smashes them together. That’s pretty awesome in and of itself, but it has the added benefit of being able to explore the universe for new particles. They have this power because they can turn one kind of matter into other kinds of matter. When particles smash together, they don’t just rearrange the pieces inside them into new configurations; the old matter is annihilated and new forms of matter are made. It’s like alchemy (we’re not kidding) at a subatomic level. This means you can almost, with some limitations, make any kind of particle that can exist without knowing in advance what you are looking for. Scientists are examining the collisions to look for evidence that some of them lead to the creation of dark matter particles.

Dark matter is a big clue that for all of our discoveries and progress we are mostly still in the dark about the nature of the universe. In terms of our understanding, we are at the same level as cave scientists Ook and Groog. Dark matter is not even in our current mathematical or physical models of the universe. There is a large amount of stuff out there silently pulling on us, and we don’t know what it is. We can’t possibly claim to understand our universe without understanding this huge part of it.

Imagine thinking all your life that you had an amazing and spacious house, and that it occupied your entire sense of everything there was. Then one day you discover it’s actually only the bottom five floors in a one-hundred-story luxury apartment building. Suddenly, your living situation just got more complicated. Twenty-seven of those other floors belong to something heavy but invisible that we’re calling dark matter. They might be cool neighbors or they might be weird neighbors. For some reason, they keep avoiding you in the hallways. Fully sixty-eight of the other floors are nearly a complete mystery. This remaining 68 percent of the universe is what physicists are calling “dark energy.” It’s the biggest chunk of reality, and we have almost no idea what it is.

We can measure the speed of a distant star from the shift in the frequency spectrum of its light using the same technique (the Doppler effect) that the police use to give you speeding tickets. The faster a star is moving away from us, the redder its light will be. Knowing how far away things are required some clever sciencing.14 For example, how do you tell the difference between a dim star that’s close by and a bright star that’s far away? Through a telescope, they look the same: like little dim points of light in the night. That was true until scientists identified a special kind of star, one that very predictably did the same thing everywhere in the universe. Because of their size and composition, these special stars grow at the same rate, and when they reach a certain size, they always do the same thing: they explode. Or to be more accurate, they implode, but the implosion is so violent, it generates a corresponding big explosion.15 This type of explosion is called a type Ia supernova. What’s useful about these supernovae is that, generally speaking, they all explode in a similar way. This means that, after some calibration, if you see one that’s dim, you know it’s far away, and if you see one that’s bright, you know it’s nearby.

Shortly after realizing this, two teams of scientists raced against each other to measure the rate of expansion of the universe. But finding supernovae is not easy because they are short-lived explosions. To catch one, you have to constantly scan the sky for stars and spot the ones that suddenly get much brighter and then dimmer so it took a while. The two teams assumed that the expansion of the universe should either be slowing down or staying the same. This is a reasonable assumption. If the universe exploded, and gravity is trying to pull everything back in, then there are only two options: either gravity wins and things get pulled back in, or it loses and everything keeps expanding steadily. When the scientists measured these supernovae and calculated the rate at which the universe was expanding, they expected gravity to be winning. That is, they expected to find that more distant stars (the ones in the past) were moving away more quickly than closer stars (the ones closer to the present). Instead, they were flummoxed to discover the opposite: that stars seem to be moving away from us more quickly now than they were in the past. In other words, the universe is expanding faster now than it was before.

We should note that the actual results showed that things were slowing down at first, but for the last five billion years, something has been pushing the bits of the exploding universe faster and faster away from one another. This driving force that’s making the universe bigger at an increasing rate is what physicists call dark energy. We can’t see it (that’s why it’s “dark”), and it’s pushing everything apart (so they call it an “energy”). And it’s such a major force that it’s estimated to represent 68 percent of the total mass and energy in the universe.

This picture captures the first photons that escaped the early formation of the universe. What’s important is that the number of wrinkles and the patterns they form in the picture are very sensitive to the proportion of dark matter, dark energy, and regular matter in the universe. In other words, if you change the proportions, then the patterns in the picture will come out differently. It turns out that for the patterns that we see in the picture you’d need about 5 percent regular matter, 27 percent dark matter, and 68 percent dark energy. Anything else would give us a different picture than what we observe.

Another way we’ve measured dark energy is by looking at the rate of expansion of the universe, which we know from the supernova standard candles. We know dark energy is pushing everything outward at a faster and faster speed. From our estimates of matter and dark matter, we can calculate how much dark energy would be needed to get that expansion, and that gives us an estimate of the amount of dark energy there is.

What’s amazing is that all of these methods agree with one another. They all reveal that our universe is roughly made up of a combination of regular matter, dark matter, and dark energy that’s 5 percent, 27 percent, and 68 percent. Even though we don’t know what each of these things are, we can say with fairly good confidence that we know how much of them there is. We have no idea what they are, but we know they are there. Welcome to the era of precision ignorance.

By comparison, dark energy makes dark matter look very simple and well understood: at least we know that it is matter. Dark energy could almost literally be anything. If a scientist from five hundred years in the future looked back in time at us, our current ideas about dark energy might seem hilarious to her, the way early men and women explaining the stars, the Sun, or the weather as being the result of gods dressed in robes seems quaint to us now. We know that there are powerful forces out there beyond our comprehension and that we have much to learn about the universe.

If the universe is expanding more and more quickly because of dark energy, it means that everything is getting farther away from us a little faster each day. As the expansion picks up speed, things that are far apart will eventually be expanding away from one another faster than the speed of light. This means that light from stars will stop being able to reach us. Already, there are fewer stars visible to us in our night sky today than there were yesterday. If you follow this expansion to its natural conclusion, in billions of years the night sky will have only a few visible stars. And, even further into the future, the night sky could be almost totally dark.

But it makes you think: if there were once more stars visible to us than there are today, what once-obvious facts are we missing because humans arrived nearly fourteen billion years after the party started?

Here’s where we stand: in the everyday business of physics exploration, we’ve discovered twelve matter particles. Six of those we call “quarks” and the other six we call “leptons.” Yet you only need three of those twelve to make up everything around you: the up quark, the down quark, and the electron (one of the leptons). Remember that with the up and down quarks, you can make protons and neutrons, and together with the electron, you can make any atom. So what are the other nine particles for? Why are they there? We have no idea.

Later on, the Greeks had the idea that everything was made of four elements: water, earth, air, and fire. This was flat-out wrong, but at least it was a step in the right direction because it tried to simplify the description of the world.

Decades ago, this table of fundamental particles was incomplete. Several of the quarks and leptons had not yet been discovered. But physicists looked at the patterns in the table and used them to go searching for the missing particles. For example, many years ago scientists knew that there had to be a sixth quark because there was an empty spot in the table. Even though it had never been found, people were so confident it existed that it was included matter-of-factly in many textbooks along with its predicted mass. After twenty years, the top quark was finally found (sort of—its mass was much higher than expected, which is why it took so long to find, and which meant all those textbooks had to be rewritten).

Quarks are unlike leptons in that they feel the strong nuclear force and they have weird fractional electric charges (+2/3 and −1/3). If you mix the up and down quarks in just the right way, you can make protons (two ups and a down quark, with charge = 2/3 + 2/3 − 1/3 = +1) and neutrons (one up and two down quarks, with charge = 2/3 − 1/3 − 1/3 = 0). That’s extremely important (and lucky) because the charge of the electron just happens to be −1. If the quarks had any more (or less) charge, then the charge of protons wouldn’t precisely balance the negative charge of the electron and you couldn’t form stable neutral atoms. Without those perfect −1/3 and +2/3 charges, we wouldn’t be here. There would be no chemistry, no biology, and no life. This is actually fascinating (or creepy, depending on your level of paranoia) because, according to our current theory, particles can have any charges whatsoever; the theory works just as well with any charge value, and the fact that they balance perfectly is, as far as we know, a huge and lucky coincidence.

Sometimes in science coincidences do happen. The moon and the Sun are vastly different in size, but by cosmic coincidence (this is one of the only times you can scientifically write “cosmic coincidence”), they appear to be nearly the same size in our sky, which allows for dramatic solar eclipses. For ancient astronomers, that must have been quite confusing and suggestive. It likely led many of them down the wrong path trying to understand if the Sun and moon were related in some way.

In addition to the twelve matter particles (we don’t count antimatter particles as unique particles)—the six quarks and six leptons—there are particles that transmit forces. For example, electromagnetic interactions are transmitted via photons. When two electrons repel each other, they are actually exchanging a photon. It’s not quite mathematically accurate, but you can think of it as one electron pushing the other away by shooting a photon at it.

There is no theoretical limit on the number of particles that could exist. There could be only seventeen particles, or there could be 100, 1,000, or 10,000,000. We know there aren’t more generations of quarks and leptons, but there could certainly be other kinds of particles. How many are there? We have no idea.

Or maybe the heavy particles are “useless” only because they can’t be used to make protons, neutrons, and electrons, which are the stable forms of the lightest particles. But the universe is mostly made of these lightest particles only because it is so cold and big. If the universe was smaller and hotter and denser, then we’d have more of the heavy particles and they wouldn’t seem so useless (but everything would be very different).

What’s crazy is that most of your body is made out of these bags of beans (protons and neutrons), which means most of your mass doesn’t come from the “stuff” you’re made of (quarks, electrons) but from the energy needed to hold your “stuff” together. In our universe, the mass of something includes the energy needed to keep that stuff together. And the mind-blowing part is that we don’t really know why. What we mean is that we don’t really know why the energy that holds the beans together affects how fast or slow something accelerates in response to a force. If you were to push on your little bag of beans, there’s no real reason why you should be able to feel that energy inside. It shouldn’t matter to you whether the beans are held together with spit or Super Glue. And yet it does. That’s one of the great mysteries of mass. Even though we can measure it, we don’t really know what inertia is or why it’s tied to both the mass of the particles and the energy that binds the particles together.

Particles—in our current theory—are actually indivisible points in space. That means that in theory they take up zero volume and they are located at exactly one infinitesimal location in three-dimensional space. There’s actually no size to them at all.32 And since you’re made of particles, that means you’re not mostly empty space, you are entirely empty space!

We like to think of particles as tiny little balls of stuff. That works for lots of thought experiments even though particles aren’t little balls. Not even a little bit. According to quantum mechanics, they are superbizarre little fluctuations in fields that permeate the entire universe. That means they obey rules that make very little sense in the tiny-little-ball model. For example, they can be on one side of an impenetrable barrier one moment and then appear on the other side the next—without passing through the barrier.34 Quantum particles can do things that seem to make no sense if you think of them in terms of things you know because they are unlike anything you have ever experienced.

How does it make sense for a particle to have zero mass? For example, the photon has exactly zero mass. If it has no mass, then it’s a particle of what? If you demand that mass is equal to stuff, then you have to conclude that a massless particle literally has nothing to it. Instead of thinking about a particle’s mass as how much stuff is crammed into a supertiny little ball, just think of it as a label that we apply to an infinitesimal quantum object.

But if electric charge means a particle can feel electrical forces (like getting repelled by other electrons), what does mass mean for a particle? Mass is the thing that gives a particle inertia (resistance to motion). But what we still don’t understand is: Why do things have inertia at all? Where does it come from? What does it mean? Who will help us in our hour of need? The answer is: the Higgs boson.

The idea that the Higgs boson might exist came out of studying the patterns of the particles that transmit forces—the photon, the W boson, and the Z boson—and asking questions about their mass. Physicists asked: Why is one of them massless (the photon) and the others (W and Z) very massive? It didn’t make sense in this particular case for this strange label that we call mass to be zero for one force particle and yet be nonzero for the others. Peter Higgs and several other particle physicists stared at this for a while until they found the solution: just make stuff up. Literally. They posited that if you add one more particle (the Higgs boson) and its field (the Higgs field) to the equations then mass as a particle label—and why some particles have it more than others—start to make sense. Roughly, the theory goes like this: imagine a field that permeates the entire universe. This field does something no other field does: rather than attracting or repelling anything, it makes it hard for particles to get going or slow down. The effect of this field is identical to the effect of having inertial mass.

The Higgs theory does explain why the force particles (photon, W, and Z) have the masses they do, but it doesn’t generally explain why the matter particles have different masses (why some interact with the Higgs field a lot and others don’t). There is probably a pattern to the masses, but it’s one that has so far escaped us. Our level of sophistication is just like Ook and Groog’s, who explained things by listing them. In the same way, our best theory of the universe only lists the masses of the matter particles as arbitrary numbers.

And, yes, because we can measure the mass both ways, and so far we have never observed one iota of difference between the gravitational and inertial masses of an object. Think about how weird that is. There’s no real intuitive reason why the two should be the same. One of them (inertial mass) is how resistant something is to being moved, and the other (gravitational mass) is how much it wants to be moved by gravity. You can do a simple experiment to confirm this. Drop two objects with different masses (like a cat and a llama) inside of a vacuum (so there is no air resistance) and you will see that they fall at the same speed. Why does that happen? If the gravitational mass of the llama is larger, then it gets pulled by a larger force from the Earth; but since the llama also has a larger inertial mass, it takes a larger force to get it moving. The two effects perfectly cancel out each other, and the cat and llama fall at the same speed. In our current formulation of physics, we don’t know why that is. We just assume it is the same. And this assumed equivalence is at the heart of Einstein’s general theory of relativity, which looks at gravity in a very different way. Rather than thinking of it as a force that acts on an arbitrary charge attached to particles and the energies that bind them, it describes gravity as the bending or distorting of space around both mass and energy. So in Einstein’s theory the connection is much more natural, but it still doesn’t tell us why it’s ...

To recap, here are the ways in which mass is weird: It’s weird because the mass of something is not just the mass of the stuff inside of it. Mass also includes the energy that binds the stuff together. And we don’t know why that is. It’s weird because mass is actually like a label or a charge (it’s not really “stuff”), and we don’t know why some particles have it (or feel the Higgs field) and others don’t. And it’s weird because mass is exactly the same whether you measure it via inertia or gravity. And we don’t know why that is either!

A good way to see the weakness of gravity is to do a little experiment that pits it against other forces. You don’t need a particle accelerator in your basement for this. Just take a standard kitchen magnet and use it to lift a small metal nail. In your experiment, that nail is being pulled down by the gravitational force of an entire planet (the Earth), and yet the magnetic force from a tiny little magnet is enough to keep the nail from falling. A tiny magnet overpowers a whole planet because magnetism is so much more powerful than gravity.

The electromagnetic force is so powerful that it will suck charges back and forth until any residual imbalance disappears. It was very early in the lifetime of the universe (when it was 400,000 years young, in the pre-papaya period) that virtually all matter settled into neutral atoms and electromagnetic forces found balance.

Gravity almost, but not quite, fits in the pattern set by the other three fundamental forces. We can think of it as a force like all the others, and we can think of mass like we think of the other charges. But gravity is much weaker and only works in one direction. This apparent inconsistency in the forces means either the pattern we have is not valid or we are missing something big.

In quantum mechanics, everything is described as a particle, even these three forces. When an electron pushes on another electron, it doesn’t use the Force or some form of invisible telekinesis to cause the other electron to move. Physicists think of that interaction as one electron tossing another particle at the other electron to transfer some of its momentum. In the case of electrons, these force-carrying particles are called photons. In the case of the weak force, particles exchange W and Z bosons. Particles that feel the strong force exchange gluons.

Quantum mechanics fails to describe gravity for two reasons. First, fitting gravity into the Standard Model requires a particle that transmits the force of gravity. Physicists have creatively called this hypothetical particle the “graviton.” If it exists, it would mean that, as you are sitting (or standing) there being pulled down by gravity, all the particles in your body are constantly throwing and receiving tiny little quantum balls with all the other particles of the Earth beneath you. And as the Earth goes around the Sun, there’s a constant stream of gravitons being exchanged between all the particles on the Earth and all the particles in the Sun. The problem is, nobody has ever seen a graviton, so this theory could be completely wrong. The other reason physicists have trouble incorporating gravity into quantum mechanics is that we already have a great theory of gravity, one that Einstein came up with in 1915. It’s called general relativity, and it works pretty well on its own. It looks at gravity in a totally different way: rather than thinking of gravity as a force between two objects, Einstein looked at gravity as a distortion of space itself. What does that mean? Einstein realized that gravity becomes simple if you stop thinking about space as an abstract concept, the invisible backdrop for all matter, and instead think of it like a dynamic fluid or flexible sheet. The presence of matter (or energy) bends space around it, changing the path of objects. In Einstein’...

According to general relativity, the reason the Earth goes around the Sun rather than flying off into space is not because there is a force that pulls it around in an orbit. It goes around the Sun because the space around the Sun is distorted in such a way that what feels like a straight line to the Earth is actually a circle (or an ellipse). In this scenario, gravitational mass is not a charge that some particles have and others don’t; rather it is a measure of how much an object is capable of distorting the space around it. As bizarre as this theory may sound, it’s been very successful at describing local gravity, cosmic gravity, and many other strange things we see out in space.

Even more problematic is that we can’t even predict what a merged theory of quantum gravity would look like. Physicists have often been able to predict particles that were later discovered experimentally (like the top quark or the Higgs boson), but so far all the theories we have that try to merge gravity and quantum mechanics fail; they keep giving nonsense results, like “infinity.” Theorists are a smart bunch (in theory), and they have some good ideas that might one day lead to a merged theory—such as string theory or loop quantum gravity—but it’s fair to say that to date the progress has been slow.

These events don’t happen on schedule and are not repeatable, but every so often, black holes get close enough to each other that they try to suck each other in. This is exactly what scientists are looking for. There are places in the cosmos where black holes are engaged in a death spiral and the collision might be generating gravitons shooting out in every direction. All we need to do is see them! It turns out that is not so easy. Even the gravitons produced by a black hole collider will be very hard to spot. The weakness of gravity means that even if a graviton passed through you, you would hardly feel it. Remember neutrinos, the ghostlike particles that can pass through light-years of lead? Gravitons make neutrinos seem like social butterflies that like to talk to everyone at the party. In fact, one calculation suggests that a detector the size of Jupiter would see one graviton every ten years even if it was near an intense graviton source.

General relativity tells us that at the heart of a black hole there exists a singularity, a point where matter is so dense that the gravitational field becomes infinite. This would be a (literally) mind-bending experience because space-time would distort you beyond any intuitive understanding. General relativity has no problem with such a thing existing, but quantum mechanics disagrees. According to the principles of quantum mechanics, it’s impossible to isolate anything exactly to a single point (like a singularity) because there is always some uncertainty. So one of the two theories has to break down in this situation. If we knew what was actually happening inside a black hole, we would have some very important clues about how quantum mechanics and gravity play together. Unfortunately, the prospects for visiting a black hole, surviving it, doing the experiments, escaping the inescapable gravitational field, and returning to Earth with the results seem daunting at this point.

Gravitational waves are the ripples in space caused by accelerating masses. It’s similar to what happens when you put your hand into a bathtub full of water and move it back and forth. Your hand will send ripples in the water down to the other end of the tub. The same thing happens when massive objects move in space. The moving mass bends space itself, creating a disturbance that can propagate like a wave.

They constructed an experiment called LIGO (Laser Interferometer Gravitational-Wave Observatory). It has two four-kilometer-long tunnels at right angles to each other, and uses a laser to measure the changes in the distance between the ends of the tunnels. When a gravitational wave comes through, it stretches space in one direction and squeezes space in the other direction. By measuring the interference of the lasers as they bounce between the different ends, physicists can measure very precisely whether the space in between has stretched or compressed. In 2016, after $620 million and decades of watching, scientists spotted their first gravitational wave. This beautifully confirmed Einstein’s picture that gravity bends space itself. Unfortunately, it doesn’t give us any insight into a quantum picture of how gravity works because gravitational waves are not the same as gravitons.

Right now, it’s quite likely that we will never visit another star system besides our own. The distances are just too far. But if we can understand the mysteries of gravity, it may lead us to understand more about how space can be bent and controlled or how wormholes can be created or manipulated. If that happens, then our wildest dreams of traveling across the universe by folding space-time could become a reality. And gravity may hold the key to that.

When space changes shape, things no longer move through it the way you might first imagine. Rather than moving in a straight line, a baseball passing through a blob of bent goo will curve along with it. If the goo is severely distorted by something heavy, like a bowling ball, the baseball might even move in a loop around it—the same way the moon orbits the Earth, or the Earth orbits the Sun. And this is something we can actually see with our naked eyes!

Light, for example, bends its path when it passes near massive objects like our Sun or giant blobs of dark matter. If gravity was just a force between objects with mass—rather than the bending of space—then it shouldn’t be able to pull on photons, which have no mass. The only way to explain how light’s path can be bent is if it’s the space itself that is bending.

Finally, we know that space can ripple. This is not too far-fetched given that we know that space can stretch and bend. But what is interesting is that the stretching and bending can propagate across our space goo; this is called a gravitational wave. If you cause a sudden distortion of space, that distortion will radiate outward like a sound wave or a ripple inside of a liquid. This kind of behavior could only happen if space has a certain physical nature to it and is not just an abstract concept or pure emptiness. We know this rippling behavior is real because (a) general relativity predicts these ripples, and (b) we have actually sensed these ripples. Somewhere in the universe, two massive black holes were locked in a frenzied sp...