by Sasha Brownsberger
figures by Abagail Burrus
Everything you have ever touched, seen, or tasted; the air you breathe; the ground on which you stand; and the constituents of your body all consist of a type of matter that is only a fraction of all that is. In light of a series of unexpected discoveries over the past half century, astronomers and physicists have determined that about 85% of all matter in the universe consists of a mysterious, invisible, and as-of-yet unidentified substance that they have dubbed “dark matter.”
Dark matter likely exists everywhere in the universe, and the Earth is perpetually flying through a diffuse cloud of this mysterious substance (Figure 1). Dark matter is not just “out there”—it is very much here and there and everywhere that you might ever go. But since dark matter interacts so weakly with other types of matter, the effects of its existence are only apparent when observed on galactic and super-galactic scales of ~1015 meters or more (for reference, the distance from the Earth to the sun, ~1.58 × 108 meters, is about 10 million times shorter). The history of scientific observations that led to the radical conclusion that the universe is filled with invisible and nearly undetectable matter is fascinating, and it illustrates how surprising results that challenge accepted theories can lead scientists in unexpected directions.
You, the confused scientist: imagining the discovery of the unexpected
Let’s step into the shoes of the first astronomers trying to understand the unexpected results of their observations by considering our own hypothetical observations.
You stand at the edge of a mysterious, vast, and clear river of an unknown liquid, tasked with formulating a theory to describe its motion. In the shallows, you find that the liquid moves predictably. You formulate a rule, based on your observations, that disturbances on the river’s surface are caused by obstacles submerged just beneath the location of the disturbance. Call it [your name]’s Theory of the Shallows (Figure 2).
Now you cast your gaze further out where you notice disturbances of the sort that your theory predicts are caused by obstructions submerged just beneath the river’s surface. Yet you notice no such obstructions. Your Theory of the Shallows, when applied to the depths, seems to predict the existence of large objects that you cannot see. Perhaps your theory is incomplete. Or perhaps there is something about the river that prevents you from seeing rocks submerged beneath its surface. Or perhaps this river contains a new type of object, entirely invisible to you, that’s able to influence its motion.
The initial realization that the universe might be filled with invisible matter resulted from a set of scientific observations that parallels this exact process. So, as we discuss some of the history of the theory of dark matter, try to keep your Theory of the Shallows in mind.
Something is wrong: the first incontrovertible evidence from rotating galaxies
Throughout the 1970s, observations of galaxies dominated by a large spiral of stars and gas provided the first evidence of a significant inconsistency with Einstein’s theory of gravity, which the majority of the scientific community found incontrovertible. By carefully observing the rotation speed of gas clouds in these galaxies, astronomers determined the strength of the galaxies’ gravitational pull at various distances from their centers. They used these results and the standard theory of gravity, which describes the relationship between how much mass a galaxy contains and how strong its gravitational pull is, to predict how much mass these galaxies must contain to produce the gravitational forces they observed. Unexpectedly, they found that the predicted mass of each galaxy significantly exceeded the total mass of all the visible objects in that galaxy. The standard scientific theory seemed to be indicating that most of the matter in these galaxies was entirely invisible.
Other astronomical observations confirmed this inconsistency. Measurements of the motions of stars in other types of galaxies consistently found that galaxies of all types contained a substantial amount of seemingly invisible mass. Further, observations of groups of galaxies had been suggesting for several decades that the galaxies were moving so fast that they would fly apart unless they were embedded in a large cloud of undetected matter (Figure 3). These observations alone, the first of which took place in the 1930s, had not been sufficient to convince the astrophysics community that there was a significant problem with the standard theory of galactic motion. But, by the end of the 1970s, the combined evidence of the anomalous motion of galaxies and galaxy clusters pointed to the same surprising reality: if the standard theory of a gravity is correct, galaxies and galaxy clusters contain a lot of mass that is missing from observations.
Accounting for the absent: three possible solutions to the missing mass problem
As the tension between the predictions of Einstein’s theory of gravity and the observed properties of galaxies and galaxy clusters become more glaring, scientists considered three broad Hypotheses:
- Einstein’s theory of gravity is incomplete, failing to accurately describe gravitational interactions over galaxy-sized and larger distances (i.e., there is no missing mass).
- These galaxies and galaxy clusters contain unobserved mass of a type that we already know (i.e., there is missing mass, and it is contained in a familiar type of matter).
- These galaxies and galaxy clusters contain unobserved mass of a type that we have never detected (i.e., there is missing mass, and it is a new type of matter).
Confronted by a problem with several possible solutions, scientists labored to determine which of these options, if any, is correct.
Earlier in the 20th century, inconsistencies between the predictions of Newton’s theory of gravity and observations of the motion of the planet Mercury helped support the new the theory of gravity created by Einstein. Thus, scientists wondered if the inconsistencies between observations and predictions so far discussed might indicate that the theory of gravity required even further modification (Hypothesis I). However, observational evidence gathered in the intervening decades strongly disfavored this solution. The strongest such evidence comes from observations of colliding galaxy clusters, which show that the regions where the gravitational force is strongest is also where the density of visible matter is lowest. Thus, there must be something unseen that is producing the observed gravitational pull.
In the 19th century, a French mathematician used anomalous observations of the motion of the planet Uranus to predict the existence and location of the planet Neptune before it was discovered, earning him renown for “discover[ing] a planet (…) at the end of his pen”. Thus, as astronomers first sought to understand what the missing matter in their observations might be, they had precedent to wonder if the inconsistencies between theoretical predictions and observations might be pointing to the existence of previously undetected astronomical objects of an already well-understood variety (Hypothesis II). However, various pieces of observational evidence—most significantly the detailed structure of the energy spectrum of the earliest observable light in the universe—have since strongly disfavored this hypothesis.
Although Hypotheses I and II were well-grounded in scientific precedent, a diversity of astronomical observations has gradually made them much less viable. Thus, what might have initially seemed like the most radical explanation for the discrepancies is now the possibility that is most consistent with a diversity of astronomical observations: most of the matter in our universe is of a variety entirely foreign to the human experience and poorly understood by scientists.
On dark matter, we are still largely in the dark
So having determined that dark matter is likely neither an erroneous prediction of an incorrect theory nor ordinary matter hidden away in a difficult-to-detect form, what exactly is it? The current answer is that we’re not sure.
Scientists are confident about some of dark matter’s properties. First, dark matter doesn’t interact with light (hence the name dark matter). Also, because there is strong evidence that dark matter existed well before fundamental particles began to form atoms, scientists have concluded that dark matter is likely one or more new type(s) of unobserved and stable subatomic particle(s). Further, by combining our knowledge of how structure in the universe evolves with efforts to detect dark matter with Earth-based detectors, scientists have constrained the possible mass and non-gravitational interaction strength of dark matter particles.
However, aside from those few very significant properties, little else about dark matter is definitively known. Scientists are still uncertain about the number of different types of dark matter particles, how they might interact with each other, whether they are detectable on Earth, and how they might change our understanding of the fundamental constituents of the universe. Scientists are still working to answer these questions by performing more accurate astrophysical observations and building more precise Earth-based dark matter detectors. But for the moment, the true nature of dark matter remains a mystery. And the history of this mysterious substance tells a story of scientific triumph and humility, demonstrating both our ability to improve our understanding of the universe and underscoring just how much more we still have to learn.
Sasha Brownsberger is a third-year Ph.D. student in Physics at Harvard University studying the astrophysical and cosmological signatures of dark matter and dark energy.