Hunting in the darkness of the universe, the hunters still don't know how their prey looks like or when and where it might pop up.

Their best clues are hidden in the chains of figures and diagrams entering computers in an inconspicuous white building of the Purple Mountain Observatory in downtown Nanjing, capital of east China's Jiangsu Province. The computers are receiving data from the Dark Matter Particle Explorer (DAMPE), 500 kilometers away in space.

Chang Jin, chief scientist of China's DAMPE program, describes the search for the missing mass of the universe: "It must be there. But we don't know if we will be lucky enough to catch it, or even if it is a bear or a rabbit."

Scientists believe that only around 5 percent of the total mass-energy of the known universe is made up of ordinary matter -- protons, neutrons, electrons -- so dark matter and dark energy make up the rest.

Dark matter, like a ghost of the universe, does not emit or reflect enough electromagnetic radiation to be observed directly, and is one of the great mysteries of modern science.

A theory of scientists who could not understand the missing mass and strangely bent light from faraway galaxies, dark matter is widely accepted in the physics community even though its existence has never been concretely proven.

Understanding dark matter could, therefore, give us a clearer idea about the past and future of galaxies and the universe, and will be revolutionary for the fields of physics and space science, Chang says.

The universe hides its secrets well. And the hunters need a good "sword."

Scientists compare DAMPE, nicknamed "Wukong" or "Monkey King", to a sword sweeping for traces left by the "ghost" of universe using the widest-ever observation spectrum and highest-ever energy resolution equipment of any probe in the world.

The satellite was successfully sent into orbit on Dec. 17, 2015. Since the launch, Chang has worried every day whether its 76,000 small detectors are working properly; whether the data are reliable; whether the storage and computing capability of the computer are enough.

DAMPE cost only one seventh of NASA's FERMI Space Telescope and one twentieth of the AMS-02 particle detector on the international space station.

Chang argues the satellite should be worth the money.

GLIMPSING THE "GHOST" 

When Chang started work at the Purple Mountain Observatory in 1992, he chose to specialize in the observation of high-energy electrons and gamma rays, because no scientist had ever done that before.

However, it required expensive equipment that China could not afford in the 1990s. So Chang developed a new and cheaper method to observe high-energy electrons and gamma rays.

He convinced U.S. scientists to include his observation method in the ATIC program, which released a balloon-borne instrument over Antarctica to measure the energy and composition of cosmic rays from the end of 2000 to the beginning of 2001.

The data analysis revealed an unexpected surplus of high-energy electrons that could not be explained by the standard model of cosmic ray origin, in which electrons are accelerated in sources such as supernova remnants and then propagate through the galaxy.

Chang believes the surplus possibly resulted from the annihilation of dark matter.

In the following years, Chang and his partners improved their equipment and method, and conducted three more observations above Antarctica.

Chang spent nearly a decade analyzing the data. His wife recalls his trance-like state at home, whispering strange numbers. When a new idea sprang to mind, he would rush to the lab to write a program to do the calculation.

Does the surplus of high-energy electrons come from annihilation of dark matter? The data of ATIC cannot exclude the interference from other celestial bodies. As long as the balloon is within the atmosphere, the high-energy particles would collide with the atmosphere and cause a lot of "noise." A probe must be sent into space for a clearer observation, Chang says.

Chang submitted his first application for developing a dark matter space probe in 2002, but received no response. He tried again in 2003, and failed again.

During those years, Chang and his team took part in the development of scientific payloads on China's Shenzhou spacecraft and Chang'e lunar probes, and won many honors.

All the time he persevered in his lonely quest for dark matter. He did countless calculations and experiments and upgraded the detailed technologies.

In 2008, Chang published an article in the authoritative journal Nature as the first author, introducing the discovery of the abnormal surplus of high-energy electrons. The discovery was regarded as one of the important research advances in physics that year.

Experts say that if confirmed, the observation would be the first evidence of annihilation of dark matter particles found by mankind. The finding sparked a global fervor to detect dark matter.

In 2011, China inaugurated a program to develop a series of scientific satellites including DAMPE. Chang's dream was being realized.

TESTING THE SWORD 

Since DAMPE was put into orbit, scientists have been calibrating the satellite in order to produce more accurate data.

"Now the payload looks perfect, but it's not enough. If the calibration goes well, the signs we seek will pop out from the data," says Chang.

Wukong is sending back about 20 GB of data a day. DAMPE advanced data process sub-system designer Zang Jingjing says all the data will be analyzed by a special computer equipped with 128 10-cored CPUs.

"After calibration, the detectors will collect more useful data and screen out signal noise. That will save a lot of time," says Zang.

"The accuracy for detecting the direction of incoming particles can reach one hundredth of a hair's width," says Zang. "That will indicate where the particles come from. If they are from the dark matter, we will know the location of the dark matter."

The prototype sample of the payloads of DAMPE was taken to CERN, the European Organization for Nuclear Research, for beam calibration three times -- the test of a good "sword". This was the first whole large detector from China taken to CERN for beam calibration.

Zang took part in all the calibrations. Each time, he lost about 5 kilograms in weight. "Although it was laborious, we were pursuing our dream."

DAMPE scientific application chief designer Wu Jian, 46, says scientists in China are relatively poor compared with doctors or company bosses. "But doing things I'm interested in can bring me a sense of satisfaction. If we can make any discovery, we will provide add to humanity's knowledge.

"We care about the gaps in science, the areas most likely to yield new discoveries. It's like a sweet potato field. If the field has been dug many times, it's difficult to harvest anything." says Wu, who worked in the data analysis team led by Nobel Laureate Samuel Chao Chung Ting in Zurich for two years.

BLAZING A TRAIL 

"I'm younger than I look," says 38-year-old Fan Yizhong, self-conscious of his gray hair.

As the deputy chief designer of the scientific application system of DAMPE, Fan and his team are responsible for analyzing the signals detected by DAMPE and identifying whether they are from dark matter or other interesting astronomical phenomena.

Born into a poor family in a rural area of Chongqing in southwest China, Fan went to a technical secondary school as his parents hoped he could get a job as early as possible to help the family. Supported by his brother, he went to college and became fascinated by astronomy after reading books by Stephen Hawking.

He got a master's degree in astrophysics at Nanjing University, and a doctorate at the Purple Mountain Observatory, and then studied in the United States, Israel and Denmark. For 10 years, he studied gamma ray bursts, a kind of supernova explosion during the death of a star.

When he applied for a job at the Purple Mountain Observatory in 2010, he was asked to change to the research of dark matter because the DAMPE program was under deliberation.

Since then he has been obsessed by dark matter. "It's really mysterious. What attracts me most is that we know almost nothing about it," Fan says.

"When can we find dark matter? Nobody can tell. Maybe tomorrow, maybe in several years," Fans says.

But he believes the satellite must make some exciting findings whether about dark matter or other astronomical phenomena.

Fan's hair turned white after the DAMPE program started. Because China has so few scientists in the field of dark matter, Fan has difficulty recruiting suitable researchers. Fan suffered insomnia for a long time worrying about his team.

Some universities offered Fan positions with better pay, but he declined. "This is China's first astronomical satellite. We are blazing a trail for China's space exploration. It is a once-in-a-lifetime opportunity." (Xinhua)