Gravitational waves are ripples in the fabric of space-time generated by some of the most powerful astrophysical events – such as exploding stars and collisions of two black holes at the centres of galaxies. Gravitational waves travel at the speed of light through the universe, unhindered by intervening mass – to gravitational waves the universe is transparent. That is why gravitational waves are the cosmic messengers that allow us to explore the so far dark side of the universe.