In this colloquium I will present the recently published results of the European Pulsar Timing Array collaboration on broadening the window of gravitational wave detections to the nano-Hertz regime. In this band, we expect gravitational wave signals of supermassive black hole binary systems in the center of merging galaxies to be present. I will give a broad overview of the history, the recent results and future prospects of gravitational wave research with pulsar timing arrays.