Carrier Long March 2-D rocket blasts off, sending into space the country's first Dark Matter Particle Explorer Satellite at the Jiuquan Satellite Launch Center in Gansu province, Dec 17, 2015. [Photo/Xinhua]

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.