The European Space Agency said construction is set to begin on its Euclid space mission to explore the "dark universe" of dark matter and dark energy.

Italy's Thales Alenia Space has been chosen as prime contractor, beginning the full industrial phase of the project, the ESA reported from its Paris headquarters Monday.

Euclid will launch in 2020 to explore the role played in the universe by dark matter, which is invisible to normal telescopes but acts through gravity to play a vital part in forming galaxies and slowing the expansion of the universe.

Dark energy, in contrast, causes a force that overcomes gravity and is accelerating the expansion of the universe being observed today.

Dark matter and dark energy are thought to account for 95 percent of the mass and energy in the universe, with "normal" matter, from which stars and planet are made, making up the remaining small fraction.

Both remain a profound mystery the scientists behind Euclid hope to begin to unravel.

"With the support of European space industry, we are a step closer to revealing the darkest secrets of the Universe," Alvaro Gimenez, ESA's director of science and robotic exploration, said.

Euclid's two scientific instruments, a visible-light camera and a near-infrared camera/spectrometer, will map the 3-D distribution of up to 2 billion galaxies spread over more than a third of the whole sky to create a map of the dark matter in the universe and study the role of dark energy.