But something was causing it. Eventually theorists came up with three sorts of explanations. Maybe it was a result of a long-discarded version of Einstein's theory of gravity, one that contained what was called a "cosmological constant. Maybe there is something wrong with Einstein's theory of gravity and a new theory could include some kind of field that creates this cosmic acceleration.
Theorists still don't know what the correct explanation is, but they have given the solution a name. It is called dark energy. More is unknown than is known. We know how much dark energy there is because we know how it affects the universe's expansion. Other than that, it is a complete mystery. But it is an important mystery. Come to think of it, maybe it shouldn't be called "normal" matter at all, since it is such a small fraction of the universe. One explanation for dark energy is that it is a property of space.
Albert Einstein was the first person to realize that empty space is not nothing. Space has amazing properties, many of which are just beginning to be understood. The first property that Einstein discovered is that it is possible for more space to come into existence. Then one version of Einstein's gravity theory, the version that contains a cosmological constant , makes a second prediction: "empty space" can possess its own energy.
Because this energy is a property of space itself, it would not be diluted as space expands. As more space comes into existence, more of this energy-of-space would appear. As a result, this form of energy would cause the universe to expand faster and faster.
Unfortunately, no one understands why the cosmological constant should even be there, much less why it would have exactly the right value to cause the observed acceleration of the universe. Another explanation for how space acquires energy comes from the quantum theory of matter.
This means it does not absorb, reflect or emit light, making it extremely hard to spot. In fact, researchers have been able to infer the existence of dark matter only from the gravitational effect it seems to have on visible matter.
But what is dark matter? One idea is that it could contain "supersymmetric particles" — hypothesized particles that are partners to those already known in the Standard Model.
Many theories say the dark matter particles would be light enough to be produced at the LHC. The rate at which the experiment detected hits from possible dark matter particles changed over the course of the year—climbing to its peak in June and dipping to its nadir in December.
This was exactly what DAMA scientists were looking for. If our galaxy is surrounded by a dark matter halo, the Earth is constantly moving through that halo as it orbits the sun—and the sun is constantly moving through the dark matter as it orbits the center of the Milky Way.
During half of the year, the Earth is moving in the same direction as the sun. During the other half, it is moving in the opposite direction. However, some loopholes exist; the particles the DAMA detector has been seeing could be something other than dark matter, something else the Earth and sun are constantly moving through. Or something else could be changing in the nearby environment.
It might be that people will come around only when several experiments start to see the same thing. Dark matter could turn out to be something stranger or more complicated than we expect.
In the space-based PAMELA experiment detected an excess of positrons—a possible result of dark matter particles colliding and annihilating one another. In the AMS experiment, attached to the International Space Station, found the same result with even more certainty. But scientists remain unconvinced, arguing that the positrons could also come from pulsars.
It seems we will need to wait until the upcoming generation of dark matter experiments is complete to get a clearer picture. Scientists have come up with several models for what dark matter might be like.
Other possibilities include particles conveniently already predicted in models of supersymmetry, a theory that adds a new fundamental particle to correspond with each one we already know. Groups of scientists are also searching for dark-matter particles called axions. Visible matter, the quarks and gluons and electrons that make up all of us and everything we can see, along with an entire zoo of fundamental particles and forces including photons, neutrinos and Higgs bosons, makes up just 5 percent of the universe.
Physicists and astronomers would like to understand, at a more fundamental level, what exactly dark matter is. Is it made up of a new type of fundamental particle, or does it consist of some invisible, compact object, such as a black hole? If it is a particle, does it have any albeit very weak interaction with familiar matter, aside from gravity? Does that particle have any interactions with itself that would be invisible to our senses?
Is there more than one type of such a particle? Do any of these particles have interactions of any sort? My theoretical colleagues and I have thought about a number of interesting possibilities. Ultimately, however, we will learn about the true nature of dark matter only with the help of further observations to guide us. Or—if we are very lucky and dark matter does have some tiny, nongravitational interaction with ordinary matter we have so far failed to observe—big underground detectors, satellites in space or the Large Hadron Collider at CERN near Geneva might in the future detect dark matter particles.
Compact or other structures akin to the Milky Way, such as the bright gas clouds and stars we see when we look at the night sky, could indicate one or more distinct species of dark matter particles that interact with one another. Or hypothesized particles called axions that interact with magnetic fields might be detected in laboratories or in space. For a theorist, an observer or an experimentalist, dark matter is a promising target for research. We know it exists, but we do not yet know what it is at a fundamental level.
The reason we do not know might be obvious by now: it is just not interacting enough to tell us, at least so far. But if dark matter has some more interesting properties, researchers are poised to find them—and, in the process, to help us more completely address this wonderful mystery.
This article is part of Innovations In The Biggest Questions In Science , an editorially independent supplement produced with the financial support of third parties.
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