The observable universe, the part of the universe that we can see, is approximately 93 billion light years in diameter. This means that the distance between us and the edge of the observable universe is about 46.5 billion light years in any direction.

What is the “observable” universe?

Now, some of you may be wondering, what is the observable universe and how does this answer the original question? The observable universe is the only part of the universe that we have access to; beyond it is more of the same, but we cannot see it because light has not had enough time to reach us from those great distances.

Not only this, but the universe is expanding, which adds a further complication to the definition of “observable.” Any question about the size of the universe can really only be answered in terms of the observable universe, at least with any hope of accuracy.

Before we delve into a discussion of how the size of the universe was determined, we have to talk about some basic assumptions in cosmology. First of all, it is important to know that the speed of light in a vacuum is constant. This is key to the definition of cosmic distances; one light year is the distance light can travel in one year.

Secondly, the idea that the universe is expanding is crucial to determining the size of the universe. As was discussed in our look into dark energy, Edwin Hubble determined that distant galaxies are moving away from us. His observations and ones that followed lead to the realization that the universe is expanding.

The last thing to know is that the rate at which the universe is expanding is increasing. Originally discovered in the late 20th century, scientists realized that the universe isn’t just expanding, it’s accelerating its growth. 

What is the distance to the edge of the universe?

Now that we have the tools to solve the problem, how do we determine the size of the universe? In order to understand the size of the universe, its critical to distinguish between the concepts of proper and comoving distances.

The proper distance between two objects in space is a measurement of their separation at some instant in time. This is the definition of distance that you are familiar with, but there is a slight complication. Since light travels at a finite speed, we can never know the actual distance to a moving object in the instant we observe it. This is because light takes time to travel from a source to us, and in this time, the source may have moved. This means that the proper distance of an object can never be observed, but it can be calculated.

The comoving distance between two objects is one that takes the expansion of space out of the equation. In a static universe, where nothing is in motion, the comoving distance and the proper distance would be the same, since there would be no expansion to account for. In our universe, however, the relationship between the constant comoving distance and the proper distance is a quantity known as the scale factor. This value changes over time to account for the variable rate of expansion of the universe. Currently, this scale factor is equal to 1. Therefore, right now, the comoving distance and proper distance are the same. 

If you are confused, that is completely understandable! They are both hard to visualize;  the comoving distance is a purely cosmological measure of distance, and the proper distance is impossible to actually see. The really confusing part is that the distances to everything we can see, from the Moon to the most distant galaxy, are facades of the actual distance. As mentioned previously, it has to do with the speed of light in combination with the expansion of space. To clear up this ambiguity, we will look at a brief example.

The most distant objects we can see are about 13.4 billion light years away. Except they aren’t actually; they are much farther away. The light from those objects traveled for 13.4 billion years, which in a static universe would mean the proper distance of the objects was 13.4 billion light years, since they could not have moved as the light traveled to us.

However, given that our universe is not static, the light left those objects when they were much closer to us, and that light took 13.4 billion years to reach us through continuously expanding space. In other words, the light left when the objects were much closer to us than they are now, but the space through which it traveled expanded to 13.4 billion light years across as it traveled through it. This gives us the illusion that the objects are 13.4 billion light years away. 

In reality, these objects are much farther away: about 32 billion light years according to the latest cosmological models. Their proper distance when the light left them was much smaller than 13.4 billion light years and now it is 32 billion light years.

Therefore, the distance to the edge of the universe appears to be approximately 13.8 billion light years, but its proper distance is actually about 46.5 billion light years.

How are these different types of distance determined?

To put these varying definitions of distance into more concrete terms, it would be as if someone threw a ball at you, and as they threw it, the space between you and the person throwing it grew. As it grows, the ball takes longer to reach you, and the person ends up being much farther away from you than when they threw the ball.

The only problem with this analogy when compared to the situation with the most distant objects is that the speed of light is the cosmic speed limit. It is the speed at which information is transmitted to us, and nothing can go faster (as far as we know).

Therefore, it would be as if the person throwing the ball was invisible and we could only determine where they were based off of the ball. If we didn’t realize space was expanding, we would assume the person was much closer than they really were, just like the illusion created by the most distant objects in the observable universe. 

So, how do we determine how far the light from any object has traveled? The short answer is by using the Doppler effect. This effect, which you might be familiar with, is what explains the change in the pitch of the siren on an emergency vehicle as it drives past you.

As it speeds towards you, its pitch increases, and as it goes away from you, the pitch drops. This is due to the velocity of the vehicle causing a shift in the frequency of the sound wave. Motion towards you causes the frequency to increase and motion away causes it to decrease. The same effect happens with light.

A distant galaxy that has some sort of recessional velocity due to the expansion of the universe will emit light that gets redshifted as it travels to Earth. This term “redshifted” refers to the decrease in the frequency of the light as caused by the Doppler effect. Since red is the lowest frequency of visible light and blue is the highest, we use the terms redshifted and blueshifted to describe the change in frequency. 

But, how do we know the frequency is shifted? Using a spectrograph, which breaks down the constituent wavelengths in light to reveal the composition of a source, scientists can find a specific signature of an element and match it to those in a distant object.

They measure the frequencies of the same element in the laboratory, where it is at rest with respect to the spectrograph. Then, when they compare this measurement to the one taken from the distant object, they can determine how fast the object is moving away from us. 

Using this information, in combination with Hubble’s law, which tells us that the distance to an object and its recessional velocity are directly related, we can determine how far the light had to travel, and, from this, calculate the age of the observed object. 

In fact, you might be wondering why there is such a large difference between the proper distance of the most distant visible objects and the edge of the universe as compared their apparent distances. This is due to the Doppler effect; for instance, an object 13.2 billion light years away is much less redshifted than one 13.3 billion light years away.

What is beyond the edge of the observable universe?

Perhaps the most mind-boggling implication of current cosmological models is that there are objects in the universe that we will never be able to see. We know that the proper distance to the edge of the observable universe is about 46.5 billion light years. We also know that it is rapidly expanding. In fact, objects that are currently beyond 13.8 billion light years (give or take a few million) away right now have a recessional velocity greater than the speed of light. 

Wait a minute. Isn’t the speed of light supposed to be the cosmic speed limit? Yes. It still is; these objects aren’t moving away from us at that speed, its just that the space between us and them is expanding so fast that it appears that they are.

Try not to mistake traditional translational velocity with the expansion of space; this is critical to understanding the “mind-boggling” part. In other words, there is a difference between an object being propelled to velocities at or above the speed of light and the distance between two objects increasing at a similar rate due to the expansion of space. The former violates the laws of physics, the latter does not.

Objects that are so far away that space is expanding faster than the speed of light lie outside of what is called the Hubble sphere. This sphere has a radius of 13.8 billion light years, which exactly corresponds with the age of the universe.

As mentioned before, many objects, particularly those that appear near the edge of the sphere, actually now lie outside the sphere. That is, their proper distance is much farther than the edge of the sphere. It’s just that the light from their distant pasts lies inside of it.

So, objects that lie outside the Hubble sphere reside in space that is expanding at a superluminal rate (faster than light). The crazy part is that light leaving these objects right now will eventually fall within the Hubble sphere and then be visible from our part of the universe (after traveling a long time, of course). 

Eventually, when the universe expands to a radius of about 62 billion light years, we will reach what is called the cosmic event horizon. This means that light emitted from objects beyond that distance will never fall inside of the Hubble sphere. This is because the space between the object and the edge of the sphere will be expanding too fast for it to reach the edge. In other words, we will never be able to see these objects. Consider yourself sufficiently “mind-boggled.”

An even more intense consequence of the expansion of the universe is that at some distant point in the future, even the closest objects to us will be so distant that they will be moving away faster than light. This means that eventually, all of our galactic neighbors may be invisible to us (assuming there is still an “us” at that point). Not to worry, of course; according to our current understanding of cosmology and dark energy, this won’t happen for a long time. 

Clearly, understanding the size of the universe is not as simple as you might have originally thought. The universe is full of unintuitive and mysterious phenomena, and the edge of it is no exception. In dealing with the immense size and time scales that come up frequently in cosmology, it is no wonder that we find ourselves as a species to be, at best, a small and insignificant part of something much grander. 

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A black hole is a source of gravity so strong that nothing, not even light, the fastest thing in the universe, can escape. More formally, a black hole is a singularity (or “tear”) in the fabric of spacetime itself. They are some of the most mysterious and paradoxical phenomena in the universe. To this day, there is a great deal that scientists do not know about black holes.

What is the history of black holes?

Black holes originated from the field equations of Einstein’s theory of general relativity in the early 1900’s. Initially, they were just a mathematical phenomenon (solutions that cause a zero denominator) in the equations. However, in the years that followed, scientists began to theorize that if a star with a sufficiently large mass died, it would crush its entire mass down into a singularity.

A singularity is the beating heart of a black hole. It is a point in space as small as an atom that contains an extreme amount of mass; some black holes are many times the mass of the Sun. The implications of an object with this property were mind-boggling. Not only would it be an object of immense gravitational pull focused at a small point in space, but it would also be powerful enough to prevent light from escaping it. Due to this, it would be completely invisible to any observer, hence its name. 

Scientists also began to discover some important properties of black holes, such as their size and the mass of a star required to create one. The former prediction implied that there would be an event horizon, a spherical boundary around the singularity through which light cannot escape. The latter determined that only the most massive stars could collapse into a black hole. 

Another key discovery about black holes was made in the 1970’s, by the late theoretical physicist, Stephen Hawking. He claimed that black holes were not immortal, as they were originally thought to be. Hawking showed that black holes slowly leaked radiation off of their event horizon that caused them to lose mass at an increasing rate over time. This quantum phenomenon became known as Hawking radiation and is now a crucial factor in determining the future of the universe. 

In the 1960’s, astronomers detected a strong source of x-rays from a star in the constellation Cygnus. This source was eventually determined to be the first black hole discovered by humans. The black hole was named Cygnus X-1 and is a black hole of about 15 times the mass of the Sun. The system it belongs to is about 6000 light years away from Earth.

Even more recently, in 2019, astronomers working at the Event Horizon Telescope (EHT) announced that they had successfully photographed a supermassive black hole in the center of a large elliptical galaxy over 55 million light years away. This black hole is much larger than Cygnus X-1, with an estimated mass of about 6.5 billion times the mass of the Sun. Although the image without context is unimpressive and blurry, it is the first of its kind and provides visual evidence that black holes actually exist.

What are some properties of black holes?

Black holes are the remnants of once bright and massive stars. As these stars begin to run out of fuel, they collapse in on themselves until the element iron is fused. At this point, the science behind nuclear fusion (the smashing of atoms together) dictates that the star begins to lose energy. In other words, up until iron, for every element fused, the star gains energy. But, as soon as iron is reached, the star no longer makes profit on the energy required to fuse it together. So, the star which has been burning for millions of years, quickly loses the battle against gravity. The outer layers collapse in on the dead core of the star at such a rapid rate that a singularity, and thus a black hole, is formed.

One of the prominent properties of black holes that scientists have predicted and observed is the way black holes interact with other matter. Specifically, when black holes “eat” matter, some of it does not fall into the hole and instead orbits around it. Eventually, this extra highly energetic matter forms what is called an accretion disk around the black hole. Scientists predict that this kind of disk is common in systems containing a black hole and one or more stars. The black hole slowly feeds off of the matter in the outer layers of the companion stars, creating an accretion disk of hot gas and dust.

Black holes come in all different sizes. Their size depends heavily on the mass of the star that collapsed to create it. However, scientists have discovered black holes with masses much greater than any one star. They have concluded that black holes can grow if they encounter one another. This type of interaction is predicted to be part of galaxy formation, as the center of galaxies is the perfect place for black holes to run into each other. Black holes of this type are called massive and supermassive black holes, depending on their mass.

Supermassive black holes at the center of galaxies seems to be an ordinary thing in the universe. Scientists now believe that there is one in the center of every galaxy; they may be a requirement for galaxies to form. The Milky Way is no exception. Astronomers have detected radio signals coming from the center of our galaxy in the constellation Sagittarius, which is an indicator of black hole activity. This, in addition to the sporadic and rapid movement of stars in the nucleus of the Milky Way, seems to point towards the existence of a supermassive black hole in the center.

What would happen if you fell into a black hole?

One of the most important consequences of Einstein’s theory of general relativity is the effect gravity has on the passage of time. General relativity says that the presence of mass or energy causes spacetime to curve, which in turn tells other masses how to move. The key takeaway from this definition is the idea that gravity not only manipulates space, but time too. Essentially, the more powerful the gravitational force on an object, the slower it’s time moves relative to some other object that’s at rest (or close to rest, like the Earth). 

Does this mean that a person near a black hole moves in slow motion? Well, close, but not quite. You see, for that person, a second feels like a normal second, but compared to an observer on Earth, they would find that for each one of those seconds near the black hole, much more time on Earth has passed. Can you use black holes to travel through time? The quick answer is sort of. One of the fundamental principles of Einstein’s theory is that while time can be slowed and sped up, it cannot be reversed. In other words, while you could travel to the future by slowing down your time relative to Earth, you cannot go to the past. Believe it or not, this is not even the strangest aspect of a journey to a black hole.

If you find yourself passing through the event horizon of the black hole, it will appear to anyone watching your perilous adventure that you are frozen on the boundary of the hole. Perhaps even crazier, in your point of view, you will watch the entire lifetime of the universe play out in front of you in a single instant. You will see everything happen that will ever happen, all at once. As far as what happens inside the black hole, there is no way to know for sure, at least not yet. Since we know of nothing that can travel faster than light, there is currently no way for scientists to probe inside of a black hole and look at the singularity.

Another consequence of the immense gravitational pull of black holes is that any object close to the event horizon will stretch out to unimaginable lengths. This process is often referred to as “spaghettification”. Each unit of distance closer to the event horizon corresponds to an inordinate change in the strength of the local gravity. It would be like if the gravity on the second floor of an office building was half of the gravity on the first floor. 

Due to this stretching and the science behind general relativity, the stretched object would also appear to get redder in color. The source of this color change is the Doppler effect, where the wavelengths of light reflecting off the object change after they pass through the gravitational field of the black hole. Unfortunately (or maybe fortunately), any human astronaut caught up in the pull of a black hole would almost certainly pass out and die before they even got close to the event horizon. The g-forces one would experience as they got close would be thousands to millions of times the livable amount. Thankfully, there are no black holes near Earth, so we are safe for now.

What do black holes tell us about the universe?

I mentioned earlier that Hawking radiation, which causes black holes to die slowly, helps scientists determine the future of the universe. While there are an enormous amount of basic factors to take into account, physicists predict that the universe will eventually be entirely composed of black holes and stray particles of light. In fact, the universe will likely live most of its lifetime without any matter; all the stars, planets, and nebulae in the universe will live and die in only a small fraction of the entire lifetime of the universe. 

Perhaps even scarier, eventually these black holes will die and the universe will slowly become a great void of nothingness. However, the fate of the universe could end up being drastically different than this if even a few of the assumptions scientists make are wrong.

Some physicists predict that black holes are just passageways to other parts of the universe and that all of the matter that they eat is tossed out the other end at some “white hole.” There has been no evidence to support this, but our knowledge of black holes is so limited, so we cannot discount it. Almost more fantastical is the claim by some that black holes lead to other universes entirely. One thing is certain: understanding the paradoxical nature of black holes could answer important questions we have about the fate of the universe and our existence in it. 

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Dark energy is a concept in modern cosmology that is best defined as the intrinsic energy in empty space, or “vacuum energy.” This energy contributes to an outward pressure that causes space to expand. In turn, the expansion creates more space, which then creates more dark energy, and thus expands space further and faster. This paradoxical interaction in which energy is seemingly created out of nothing is what scientists theorize is responsible for the accelerating expansion of the universe. 

How did the idea of dark energy come to be?

The history of dark energy originates with Albert Einstein’s famous general theory of relativity. The theory is the most accurate descriptor of gravity and its functionality. It accomplishes this feat through what are known as the Einstein field equations. These equations explain that gravity is the effect of massive and energetic objects curving the fabric of space (and time).

In 1917, after Einstein first developed the equations, he included an additional term called the cosmological constant. The sole purpose of this constant was to ensure that the equations described a static universe. We now know that the universe is not static; it was created in a massive explosion, the Big Bang. This term now represents dark energy, as seen in the image below.

Einstein, however, formulated the field equations before the Big Bang theory became the prevalent theory of the origins of the universe. Therefore, the cosmological constant was initially meant to act as an outward pressure, or “negative gravitational force” in empty space, thus preventing massive objects from crushing space in on itself. Einstein calculated the constant in such a way so that this outward pressure would only be enough to keep the universe from changing size. 

In 1929, Edwin Hubble, an American astronomer for whom the famous space telescope is named after, discovered that galaxies outside our own were moving away from us. He noted that the farther away the galaxy was, the faster it was moving away. This indicated to Hubble and the scientific community at the time that the universe was expanding. To clarify, consider for a moment that the universe is a chocolate chip cake with chips representing galaxies. When the cake is baked in an oven, it expands. Each chocolate chip moves away from the others as the space between them expands.

This is exactly what happens with the universe; the space between galaxies expands, with the effect being that the farther away a galaxy is, the faster it is moving away from us. Hubble formalized this principle in what is now known as Hubble’s law. 

After Hubble’s discovery, Einstein retracted the idea of the cosmological constant and considered it one of his biggest blunders. From then on, it was accepted that the constant had a value of zero. However, in 1998, Einstein’s supposed blunder became relevant once again when scientists discovered that the rate at which the universe was expanding was accelerating. His idea of vacuum energy was the perfect explanation for an accelerating universe. Since then, scientists have calculated a positive, non-zero value for the cosmological constant to reflect the principle that empty space has energy. 

Why is understanding dark energy so important?

Why is solving the dark energy mystery so critical in modern science? Recent studies of the cosmic microwave background radiation have revealed the mass and energy content of the entire universe. Scientists believe that about 70% of the entire universe is made up of dark energy. Only around 30% of the universe is matter, and about 70% of that figure is dark matter.

Therefore, every galaxy, star, planet, and nebula only account for about 5% of the makeup of the universe. That alone should reveal why it is crucial to modern science that we understand what dark energy is and whether or not it actually exists. 

Dark energy will dictate the fate of the universe. If the cosmological constant has a positive value, then empty space has a positive energy density and therefore exhibits an “anti-gravitational” outward pressure on the universe, causing it to expand. As space expands, more space is created, and therefore the universe has more dark energy with which to push out on itself.

The cosmological constant may not be a constant; this type of dynamic dark energy is often referred to as quintessence. If this value increases over time, then the expansion will accelerate infinitely, and we are left with a “run-away” universe, which will expand until planetary systems and even subatomic particles rip themselves apart; this scenario is commonly known as the Big Rip. If the expansion slows to some constant rate, then objects will move away from each other until they cannot see each other and galaxies are left on their own, isolated from the rest of the universe; this is the Big Freeze.

If the expansion completely stops, then, depending on the critical density of the universe, everything may collapse in on itself in a “reverse Big Bang,” called the Big Crunch. Understanding the eventual fate of the universe depends greatly on our understanding of dark energy. 

Unfortunately, as was the case with dark matter, dark energy is not a perfect theory. While it works perfectly in the field equations, it does not match up with the value for vacuum energy predicted by quantum field theory (QFT). This theory is the modern model of quantum mechanics, a branch of physics that deals with atomic and subatomic interactions.

The Einstein field equations and general relativity as a whole are a classical theory; it does not take quantum theory into account. The discrepancy between a quantum and a classical theory can be seen in the difference of the prediction made by QFT and the cosmological constant. The quantum value is about 10¹²⁰ times larger than the value given by Einstein’s equations. This extreme difference has been dubbed by physicists the “worst theoretical prediction in the history of physics.” 

So, the value is wrong, does this mean that dark energy is not real? Not necessarily. It is quite possible that the field equations need revision to include quantum mechanics, but this does not mean that the cosmological constant and the idea of dark energy would be removed entirely. Perhaps a greater problem for the existence of dark energy lies in our fundamental understanding of gravity. 

General relativity, although the most accurate model of gravitation, does not explain what gravity is, only how it acts. The Standard Model of particle physics, one of the greatest achievements of quantum mechanics, dictates that there must be a force carrier particle, or boson, for each fundamental force in the universe. For the electromagnetic force, the boson is the photon, commonly known as a particle of light.

The boson for the strong nuclear force, the force that binds protons and neutrons together in atomic nuclei, is the gluon. For the weak nuclear force, the force that is responsible for the radioactive decay of atoms, the force carrier particles are the W and Z bosons. Scientists have not discovered the boson for gravity, but it is hypothetically known as the “graviton.” The discovery of this particle will provide valuable insight into the mysterious behavior of gravity. 

As was discussed regarding dark matter, gravity may work differently on large scales, perhaps due to the strange quantum behavior of the theorized graviton. It may be that some gravitational interaction on the distance scales as large as those seen between galaxies is what causes the accelerating expansion of the universe. In order to properly unveil the mysteries of gravity, it will likely require a revolution in quantum mechanics. Such a revolution would undoubtedly provide us with insight into the mystery of dark energy and unveil some of the deepest secrets of the cosmos.

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In essence, dark matter is a type of matter that is “invisible” to all forms of light; it does not interact with any of the electromagnetic spectrum (i.e. x-rays, visible light, radio waves). The only interaction it has with the rest of the universe is via the force of gravity.

Although this strange form of matter is purely theoretical at the moment, it serves as a solution to other scientific problems that have been observed in the universe. The question of whether or not dark matter actually exists is one of the greatest mysteries in modern science.

Where did the idea of dark matter come from?

The first question you might ask is if it is not visible or measurable, then how do we know it is there or how it functions? The most direct evidence of dark matter is in spiral galaxies, specifically in the outer regions of their spiral arms. As far back as the early 20th century, astronomers noticed that the stars and gas in these outer regions did not conform to the known laws of gravity and orbital motion.

In 1933, Fritz Zwicky, a Swiss-American astronomer, was the first scientist to propose that there was some sort of non-visible matter responsible for the anomalies found in spiral galaxies. He called this hypothetical matter dunkle materie, which literally means “dark matter” because he could not see it! Since then, dark matter has proven to be as mysterious as it was when Zwicky first thought of it.

In order to explain the peculiar behavior of objects in the outer reaches of spiral galaxies, let us treat our own solar system as a tiny galaxy model, where the Sun is the nucleus of the galaxy and the planets represent stars in the spiral arms. The innermost planets, Mercury and Venus, move much quicker around the Sun than more distant planets, such as Saturn or Neptune.

The law of gravity dictates that the rate at which a body orbits the center body (in this case, the Sun) is directly related to its distance from said center body. Specifically, the farther away a planet is from the Sun, the slower it moves in its orbit.

We know this to be true; in fact, scientists know to extreme precision how long it takes for each planet in the solar system to revolve around the Sun.

The same dynamic was expected for galaxies. In the spiral arms, far from the massive nucleus at the center, stars and gas should move slower the farther out they are. However, what astronomers actually discovered was that this was not the case! In fact, astronomers observed countless galaxies and found that the orbital speeds of objects in the outer regions were nearly the same no matter how far away from the center they were.

How can this be true? Is our fundamental understanding of gravity flawed? While there is not currently a definitive answer to this problem, scientists prefer the idea of dark matter over searching for a refined theory of gravity, at least for now.

So, how does dark matter fit into this picture? As was implied earlier, the law of gravity states that all bodies that have mass attract other bodies with mass, and that the magnitude of that attraction depends on how massive the bodies are. If there was some sort of “gravitational glue” forcing the stars and gas in the spiral arms to move together, that would explain their strange behavior. This is exactly what scientists think dark matter is: undetectable particles that have a gravitational pull on the objects around them.

And the math checks out. By analyzing computer models, scientists have shown that the existence of high concentrations of dark matter in the outer regions of spiral galaxies ensures that the predicted motion of objects in those regions matches what is actually observed.

The most prevalent theory for dark matter suggests that the best candidates for particles that make up dark matter are weakly interacting massive particles (WIMPs); these are particles about 100 times more massive than electrons that do not interact with light at all, only with gravity. Unfortunately, there is currently not that much evidence to support the existence of WIMPs, as they are extremely hard to detect.

Recent efforts to find and understand WIMPs have involved various types of ground-based experiments, such as colliding particles at high speeds in order to break them down into their constituent particles. Other types of experiments have made significant progress in the search for WIMPs. For example, the recent XENON1T project involved the use of subterranean hyper-sensitive particle detectors to look for specific results from rare radioactive decay events found in exotic elements such as xenon.

Why is solving the dark matter mystery so important?

Dark matter is one of the great mysteries of modern science. Many fields of study, such as cosmology and galactic astronomy, rely on models of the universe that include dark matter. Future development of these fields, as well as countless others, hinge on the knowing whether or not dark matter exists.

So, why can’t scientists just assume that it exists and move on? One of the key principles of science is to seek truths about the natural world; ignoring a problem as enormous as dark matter would seem to fly in the face of that principle. More importantly, glossing over the dark matter mystery would potentially leave scientists ignorant of a large portion of the universe.

Studies of the cosmic microwave background (CMB) have allowed scientists to quantify and understand the matter and energy content of the entire universe. Simply put, scientists have made observations of the residual radiation left over in the universe from the Big Bang (the massive explosion that created everything) which has granted them insight into what the universe is actually composed of.

The current model of cosmology indicates that dark matter makes up around 85% of all matter in the universe. That’s right! All of the matter that you can see makes up only a small fraction of what is actually out there. In other words, every star, nebula, planet, and galaxy in the universe only accounts for about 15% of the total matter out there! 

While modern cosmologists make the assumption that dark matter exists in their current models of the universe, it is not the only theory that explains the strange behavior of spiral galaxies. The most accepted alternative hypothesis to the dark matter problem is that our understanding of gravitational interactions on scales as large as those seen in galaxies is flawed. It is not completely unreasonable to assume that the current theory of gravity, Einstein’s general relativity, is outdated. His theory of gravity superseded Newton’s work on gravity about a century ago, as the latter was unable to explain certain phenomena in the natural world.

Scientists already know that Einstein’s version of gravity does not correctly explain interactions on the smallest scales in the universe; in the quantum realm. Perhaps it will require a revolutionary scientific mind to make the mental leap required to reveal the answer to this problem.

However, as there is currently no substantial evidence refuting the existence of dark matter, there is no need to rewrite physics textbooks just yet. Either way, you shouldn’t worry about a flawed theory of gravity affecting your everyday life; gravity will still work the same as it always has.

If our unrelenting drive to understand the cosmos is an indication of anything, it’s that finding the solution to the dark matter problem is inevitable. And when scientists finally do solve the dark matter puzzle, it will be one of the greatest scientific feats of our generation, or even the entire modern era.

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