What is Dark Matter?

Credit: NASA Goddard Flight Center

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.

Dark matter populates the outer reaches of spiral arms in galaxies, like this barred spiral galaxy, NGC 1672. Credit: NASA/ESA

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.

In an effort to find clues about dark matter, scientists “smash” particles together at extremely high speeds in facilities such as the Large Hadron Collider, at which the famous Higgs boson was discovered.
Credit: ESA/CERN

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! 

A computer model of the large-scale structure of the universe. Each filament of light contain millions of galaxies and galaxy clusters, all of which only account for about 85% of the total matter in the universe.
Credit: Virgo Consortium

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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