The science of cosmology has had a spectacular run. Over the past few decades, cosmologists have carried out measurements and observations that have enabled us to reconstruct our universe’s past in incredible detail. We can now say with great confidence that we understand how and why our universe evolved over the vast majority of its history. From this perspective, our universe looks more comprehensible than ever before.

And yet, not all is understood. Despite our considerable efforts, there remain essential facets of our universe that we simply do not know how to explain. Perhaps the most famous of these mysteries is that of dark matter. Modern measurements have determined the amount of matter in our universe to very high degree of precision, and it is much more than exists in the form of atoms. After decades of debate, we are now confident that most – about 84 percent – of our universe’s matter does not consist of atoms or of any other known substances, but of something else that does not appreciably radiate, reflect, or absorb light. For lack of a better name, we call this mysterious stuff dark matter. But naming something is very different from understanding it.

A decade ago, many cosmologists – including me – thought we had a pretty good idea of what the dark matter likely consisted of. The arguments we made were predicated on how we thought this substance was formed during the first fractions of a second after the Big Bang. The quantity of dark matter particles produced in the early universe that then survived the conditions of the Big Bang, we calculated, should depend on how much those particles interact with themselves as well as with ordinary forms of matter. Based on our calculations, we were led to think that the dark matter must interact through what is known as the weak nuclear force, or through some other unknown force that is roughly as powerful. We called such particles WIMPs – weakly interacting massive particles – and they were our best guess for dark matter’s identity.

If the dark matter is indeed made up of WIMPs, then it should be possible to conduct experiments that could directly detect and measure individual particles of this substance. With this goal in mind, a small army of physicists began to build ultra-sensitive dark matter detectors, deploying them in deep underground laboratories where they would be protected from the most distracting kinds of cosmic radiation. At the time, the odds seemed quite good that this approach would succeed. In fact, I made a bet in 2005 that dark matter particles would be discovered within a decade. I lost that bet. From a technological perspective, these experiments performed beautifully. Yet no signals appeared. Adding insult to injury, the Large Hadron Collider also began its operation during this time, finding no signs of dark matter. From these experiments, we learned that the dark matter is far more elusive than we had once imagined.

Our failure to detect particles of dark matter has had a palpable effect on the scientific community. Although it remains the case that a discovery could still plausibly lie right around the corner, most of us studying dark matter today will acknowledge that many of our favorite dark matter candidates should have been detected by now. This has driven the field to redirect its efforts toward new and sometimes very different ideas, ushering in an explosion of theoretical work related to dark matter and its nature.

One newly popular idea is that the dark matter might not be alone, but instead could be one of several kinds of particles that constitute what is known as a “hidden sector.” The particles that make up such a hidden sector could interact among each other, but almost never with any of the known forms of matter, explaining why they have been so difficult to detect in underground experiments or to produce at the Large Hadron Collider. The particles that make up a hidden sector could have evolved and interacted in the early universe in any number of potentially complex ways, even experiencing forces that we have never witnessed. Particle physicists have proposed many theories in which the interactions between multiple kinds of hidden matter can lead to the viable production of dark matter in the early universe. In fact, it’s been quite easy for particle physicists to come up with viable hidden sector theories that behave in this way.

Another possibility has less to do with the dark matter itself, and more to do with the space that it occupied during the first fractions of a second after the Big Bang. When we use the equations of general relativity to calculate how fast space should expand, we take into account all of the known forms of matter and energy, including all of the kinds of particles that we have observed at the Large Hadron Collider. But it’s entirely plausible that other forms of matter were present in the early universe that we don’t know about yet. If this was the case, then our universe may have expanded quite differently from how we currently envision. And if the early universe expanded either faster or slower than we currently expect, this would change how the dark matter particles interacted during this era, as well as how much of this substance would have survived these critical moments.

The range of possibilities for how our universe may have expanded and evolved during its first second is enormous. Unknown forms of matter and energy may have increased the rate of expansion, but it’s possible that far stranger things occurred during these first moments as well. Perhaps our universe experienced a brief and sudden burst of expansion, or underwent a dramatic phase transition at some point during its first second. Alternatively, there may have been a population of particles that decayed, heating our universe and altering its evolution. The possibilities abound. Such events could have dramatically impacted how the dark matter was formed and interacted during our universe’s first moments. If we were to learn one day that such an event really did take place, this would almost certainly change our expectations about the nature of dark matter and the kinds of experiments that we would need to carry out in order to detect it. It might even explain why the dark matter has remained so elusive for so long.

The remarkable progress of underground dark matter detectors and the Large Hadron Collider has thrown the field of cosmology into a state of major disruption. Dark matter, it seems, is quite different from what most of us had once thought. The stubborn elusiveness of dark matter has forced us to abandon many of our favorite theories and to consider some radically new ideas about this substance and the conditions under which it was formed in the first instants after the Big Bang.

By striving to discover the nature of dark matter, we hope not only to identify the particles that make up most of the matter in our universe, but also to learn about the earliest moments of our universe’s history. In this sense, dark matter provides us with a window into the Big Bang. I have no doubt that these earliest moments hold incredible secrets; but our universe holds its secrets closely. It is up to us to coax those secrets from its grip, transforming them from mystery into discovery.

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