I am a theoretical physicist whose research lies at the unique interface of gravitational-wave physics, particle physics and cosmology. The target of my research is to solve the very fundamental open questions in the evolution of our Universe, namely the mysteries of dark matter and dark energy (or possible modifications of general relativity). To address this tremendous challenge, however, I do not only use cosmological tools and astrophysical data, but also a new, unprecedented window into the Universe: gravitational waves. The first ever direct detections of gravitational waves from merging black holes and neutron stars by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo detector have opened a fundamentally new window into the Universe. In my research, I turn merging black holes and neutron into cosmic labs to test the very foundations of general relativity and unveil the unknown particle nature of dark matter.