The existence of gravitational waves is a basic feature of general gravity but even Albert Einstein seriously doubted they actually exist. There is now indirect evidence that there are astrophysical sources, such as binary pulsars, which generate gravitational waves, but their strength here on Earth was too small to be detected.
However, after using advanced technologies to build large laser interferometric detectors (LIGO), detection sensitivity was significantly increased and, recently the profile of the first directly observed gravitational waves emitted by the spiral and merger of two black holes was detected. It revealed surprisignly massive stellar black holes, which merged together a billion years ago.