Euclid is a satellite that the European Space Agency will launch around 2020. The main goal of Euclid is to map the position and distance (or rather redshift) or 30 to 50 millions galaxies and to record the image of another two billions. With this unprecedented wealth of data, cosmologists will be able to reconstruct the expansion rate of the universe and the clustering of galaxies down to distances of two thousands Megaparsec. Comparing these results with predictions of theoretical models, they will get much closer to determining the properties and nature of dark energy and dark matter and the large-scale structure of space-time.
The Euclid collaboration, composed by over a thousand scientists from 15 different countries (all Europeans plus USA), including me, has recently published a review of the main characteristics of the mission.