For thousands of years, we have been looking at the Universe through our eyes. But over 99% of the Universe is dark and will never be observed with electromagnetic waves. Since September 14, 2015, everything has changed: gravitational waves have been discovered! We have been given a new sense and can finally hear the dark side of the Universe. The first sounds we heard came from unexpectedly heavy black holes. Gravitational waves have now become a new branch of astronomy. Hundreds of such events are now being recorded almost daily, with frequencies ranging from a few hertz to a few kilohertz. And in 2035, with LISA, an observatory in the millihertz range, we will also be able to observe low-frequency sources. No one knows what other dark secrets are waiting for us out there.
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