As the most compact object in the universe, black-holes can act as ultra-highly energetic accelerator of colliding particles.
Gravitational wave. Matter curves the spacetime, whose vibration creates ripples of spacetime, though usually extremely tiny.
Dark energy. Beyond Einstein's relativity, there is still an accelerated expansion of the universe. People tries to solve the vaccum energy problem in various ways.
As the most compact object in the universe, black-holes can act as ultra-highly energetic accelerator of colliding particles.
Related paper: Chinese Physics C,Vol. 44, No.6 (2020) 065104, as follows.