Backgrounder: Will we finally see the cosmic ghost?

BEIJING, Nov. 30 (Xinhua) -- It constitutes most of the mass in the universe, but we've never seen it. Now China's first dark-matter detection satellite, "Wukong", is helping scientists lift the "cloak of invisibility" from dark matter.

The initial detection results of cosmic ray electrons and positrons based on observation by Wukong, or the Dark Matter Particle Explorer (DAMPE) launched in December 2015, were published in the latest issue of the academic journal, Nature.

Scientists still need time to discern whether the signals detected by DAMPE come from dark matter or other astrophysical phenomena.

But what is dark matter?

Comparing the universe to a cosmic pie made up of three parts, scientists calculate that normal matter, such as atoms, stars, galaxies, trees, rocks and dust, accounts for just under 5 percent. About 26.8 percent is dark matter and 68.3 percent dark energy, both of which are invisible. Everything we experience is really a tiny fraction of reality.

HOW WAS DARK MATTER DISCOVERED?

Dark matter cannot be seen or touched. The ghost-like material passes right through you as if you don't exist. But why are scientists so sure it does exist?

In the early 1930s, Swiss astrophysicist Fritz Zwicky was studying the Coma galaxy cluster. His calculations implied the visible mass didn't generate enough gravity to hold the galaxies together. Without another kind of matter that humans can't see, galaxies would have flown away from each other. So he concluded there must be something nobody had detected before to hold them in place. He coined the term "dark matter" to describe it.

Zwicky was decades ahead of his time. His deduction didn't raise attention in academic circles until the 1970s, when U.S. astronomer Vera Rubin and her colleagues confirmed it by studying galaxy rotation. They also found single galaxies, not just clusters, have more mass than the observable light suggested. The work of Rubin and her team helped to firmly establish the notion of dark matter.

Since then, astronomical discoveries such as rotation curves of disk galaxies, X-ray observations of clusters and the gravitational lensing effect have all suggested the existence of dark matter.

HOW DO WE SEARCH FOR THE COSMIC GHOST?

Dark matter, which does not emit or reflect any electromagnetic radiation, cannot be seen directly by optical or electromagnetic observation equipment. Currently, scientists mainly use three methods to detect dark matter.

The first method involves extremely sensitive apparatuses on Earth based on the idea that dark matter should travel through the material of a detector until it hits a nucleus. The interaction would then provide a small amount of recoil heat or energy that can in principle be measured. Since the signal would be very weak, such experiments are usually conducted deep underground sheltered from cosmic rays and other interfering signals.

China runs the world's deepest underground dark matter lab in the southwest province of Sichuan, some 2,400 meters below the surface.

In the second method, scientists collide high-energy particles with the aim of creating dark matter. Although the dark matter particle cannot be observed directly, it will take away energy. By analyzing the missing energy, scientists can infer the properties of the dark matter particle.

The Large Hadron Collider (LHC), at the CERN (the European Organization for Nuclear Research) near Geneva, is believed to have the potential to create dark matter.

Another promising strategy is to look for signals from dark matter annihilation. Such searches look for particles such as electrons and their anti-particles - positrons - or pairs of photons, which are created after the annihilating dark matter particles disappear.

The Alpha Magnetic Spectrometer (AMS02), developed by scientists led by Nobel Laureate Samuel Chao Chung Ting and installed on the International Space Station (ISS) in 2011, the CALorimetric Electron Telescope (CALET), developed by the Japan Aerospace Exploration Agency for the ISS in 2015, and China's DAMPE all use the third method.

"This is like tracking the 'son' of dark matter - if you cannot find the father, you go to the son and you can understand at least some properties of his father," said Chang Jin, chief scientist of DAMPE.

THE INVISIBLE WORLD

Although scientists don't yet know what dark matter actually is, they learned something by observing how it affects ordinary matter through gravity.

Dark matter is electrically neutral. Its density is small. Dark matter should have been created out of the Big Bang. It's been working since the dawn of time. It triggers the birth of galaxies and keeps them from falling apart. In a halo form around galaxies, dark matter doesn't just hold them together; it might have sparked them into life.

Scientists know more about what dark matter is not than what it is. They know dark matter is not just clouds of normal matter without stars, because it would emit particles that can be detected. Dark matter is not anti-matter, because anti-matter produces unique gamma rays when it reacts with normal matter. Dark matter is not made up of black holes, which are very compact objects, while dark matter seems to be scattered everywhere.

Dark matter is probably made up of a complicated exotic particle. Many physicists have their favorite dark matter candidates, among which WIMP, the acronym for "weakly interacting massive particle", is promising.

Even if dark matter consists of an elementary particle, scientists don't know its mass, if it has any non-gravitational interactions, or how it was created in the early universe.

That's what makes dark matter one of the great mysteries of science.

Discovering its true nature has become the most pressing question in cosmology, perhaps in all physics. Any discovery in this area could be as significant as heliocentric theory, the law of gravity, the theory of relativity and quantum mechanics.

Exploration of dark matter could give us a clearer idea about the past and future of galaxies and the universe, and will be revolutionary for the world of physics and space science, said Chang.

When scientists discovered quantum mechanics in the early 1900s, many at first thought it had no use. Now quantum mechanics is a pillar of modern physics and everything is related to it.

"Only when we understand the nature of dark matter, can we find how it will change the future. But the development of fundamental physics will definitely boost science and technology," said Chang. Enditem