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China is joining the global competition on a scientific frontier－the search for dark matter－with new research instruments and facilities in place that heighten anticipation for answers.

A scientist checks equipment designed for dark matter research at Jinping Underground Laboratory. [Photo/Xinhua]

On Thursday, the Chinese Academy of Sciences launched the country's first dark matter satellite, which will be in service for three years to search for signs of the mystery substance, as well as to study the origin of cosmic rays and observe gamma rays.

"We hope we are lucky enough to become the first team in the world to find dark matter," said Chang Jin, deputy director of the academy's Purple Mountain Observatory.

Dark matter is extremely difficult to measure because it seldom interacts with anything, other than generating gravity. Most researchers trying to measure it using electronic or magnetic signals have reached a dead end.

However, Chang believes that when dark matter particles collide and annihilate each other, they generate other particles that can be detected－electrons, antiprotons and gamma rays. He proposed the idea when he joined the Advanced Thin Ionization Calorimeter project sponsored by NASA, the United States space agency, in 1998.

Between 2000 and 2003, the team launched a balloon-borne instrument over Antarctica to detect comic rays at low altitudes. It found excess electrons that may have been emitted by dark matter annihilations somewhere in space, perhaps 3,000 light years from Earth.

The discovery was reported by the journal Nature in 2008, as exciting indirect evidence for the existence of dark matter.

After the initial success, Chang proposed launching a dark matter satellite－one with more advanced detectors and higher resolution－in space rather than in the atmosphere. In space there is stronger cosmic radiation and less interference.

"The indirect evidence can give us a glimpse into dark matter, just as we can infer the nature of a father by seeing his son," Chang said.

Besides the attempt to find indirect evidence, Chinese scientists are doing parallel experiments to find direct evidence.

In 2010, the Jinping Underground Laboratory－a dark matter laboratory beneath 2,400 meters of rock－was put into operation by two groups of experimenters: the international project PandaX led by Shanghai Jiao Tong University, and the CDEX project from Tsinghua University.

Using approaches different from those of astrophysicists, these groups try to observe dark matter directly instead of tracing it via the particles emitted during annihilation events.

"Imagine that you are driving your car in the pollution haze, and each time a pollutant particle hits into your car it makes a weak sound. What we are doing in the underground laboratory is trying to figure out these sounds," said Liu Jianglai, a professor at the Shanghai school and a member of the PandaX team.

How difficult would that be? In Liu's word, it's like "trying to hear the sound of a mosquito waving its wings 30 meters away while you are sitting in the front row of a concert".

The laboratory was built deep underground to help researchers avoid as much interference as possible.

"We are close to the edge of dark matter, and we are eager to make sure it is Chinese people who give the answer to this cutting-edge scientific question," Ji Xiangdong, a professor at Shanghai Jiao Tong, was quoted as saying earlier by Xinhua News Agency.

Bi Xiaojun, a researcher from the science academy's Institute of High Energy Physics, said the decadeslong search has been narrowing the range in which to look for dark matter.

"This is like searching for a missing plane at sea. We search the energy ranges in which dark matter could possibly exist, and rule out those we already checked," he said. "Now we are working on a range with high possibilities. If we do not find dark matter here, maybe we will be lucky in the adjacent waters."

No matter what is discovered by the new instruments, the physics field is ready to embrace revolutions in the theory of dark matter, said Wu Xiangping, an astrophysicist at the National Astronomical Observatory of the Chinese Academy of Sciences.

"Whether or not you believe in the existence of dark matter, the development of physics has reached a phase that it needs an update. Either a new theory or new phenomena will breathe in new life," he said.

Physics has evolved from Newton's classical mechanics to quantum mechanics and the theory of relativity.

If we were able to further extend our vision to new matter, a revolution in physics will follow.

"Now is the time," he said.

What on Earth, or off, is dark matter?

When astronauts look at Earth from space at night, they can see major clusters of city lights that identify urban areas like Paris, Chicago and Shanghai.

But the brightest areas are not necessarily the most heavily populated. Dark spots in the interior of China and India, are also teeming with activity.

Likewise, looking outward from Earth it's easy to assume that the dark spots in the heavens are empty. Looking into the vastness of the universe from here, humans since Galileo thought there was a lot of nothing out there.

That theory thrived as mankind gradually expanded the view from visible light to a broader range of wavelengths. More "city lights" could be seen with more advanced telescopes, but empty space was still thought to the main feature of the cosmos.

That belief abruptly changed in the 1930s, however, when astronomers Fritz Zwicky and Sinclair Smith measured the speed of galaxies in the large galactic clusters Virgo and Coma. They discovered that all galaxies were moving 10 to 100 times faster than they were supposed to be, given their estimated mass.

That would mean something unseen was generating additional gravity-and the theory of dark matter and dark energy was born.

In the 1970s, stronger evidence for the existence of dark matter and dark energy showed up. Scientists such as Vera Rubin and Ken Freeman analyzed the rotational curves of some spiral galaxies, including our own Milky Way, and found that stars were moving faster around the galactic core than the velocity based on their observable mass. That indicated the presence of something with mass we cannot observe-not only with the naked eye but also with any of the current tools of modern science.

So, what on－or off－earth is dark matter or dark energy? It is too early to say for sure.

The only thing physicists feel certain about it is that this mystery matter makes up 95 percent of the universe and is waiting to be explored.

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