Abstract: Every time that we have looked further into the Universe, we have found fascinating surprises. Most notably, we observe that an omnipresent dark energy drives the Universe to expand ever faster, and that its large-scale structures hold together only because of an invisible dark matter. We do not fully understand how cosmic structures form, and even if our laws of gravity are valid at all scales. Gravitational waves (GWs) – ripples in space and time produced by extreme gravitational systems in the cosmos such as binary black holes – offer a unique opportunity to answer these fundamental open questions.