"The discoveries that the expansion of the universe is accelerating due to an unknown “dark energy”and that most of the matter is invisible, highlight our lack of understanding of the major constituentsof the universe. These surprising findings set the stage for research in cosmology at the start of the21st century. The objective of this proposal is to advance observational constraints to a level where we can distinguish between physical mechanisms that aim to explain the properties of dark energy and the observed distribution of dark matter throughout the universe. We use a relatively new technique called weak gravitational lensing: the accurate measurement of correlations in the orientations of distant galaxies enables us to map the dark matter distribution directly and to extract the cosmological information that is encoded by the large-scale structure.To study the dark universe we will analyse data from a new state-of-the-art imaging survey: the Kilo-Degree Survey (KiDS) will cover 1500 square degrees in 9 filters. The combination of its large surveyarea and the availability of exquisite photometric redshifts for the sources makes KiDS the firstproject that can place interesting constraints on the dark energy equation-of-state using lensing dataalone. Combined with complementary results from Planck, our measurements will provide one of thebest views of the dark side of the universe before much larger space-based projects commence.To reach the desired accuracy we need to carefully measure the shapes of distant background galaxies. We also need to account for any intrinsic alignments that arise due to tidal interactions, rather than through lensing. Reducing these observational and physical biases to negligible levels is a necessarystep to ensure the success of KiDS and an important part of our preparation for more challenging projects such as the European-led space mission Euclid."