With the first direct detection of gravitational waves in 2015, the era of gravitational wave astronomy has begun. But on Earth, observations are limited to objects with masses a few 10’s that of the Sun, which produce high-frequency signals. Sources with much larger masses, such as the mergers of massive black holes at the centres of galaxies, produce signals at much lower frequencies, undetectable on Earth. LISA will detect such signals and will deliver exciting new science. It will allow us unique insights in our universe – its beginning and evolution, its population, maybe the big bang and so far unknown components.