LIGO is an international network of laboratories that detect the ripples in spacetime produced by the movement of large celestial objects like stars and planets.

Source: https://indianexpress.com/article/explained/explained-sci-tech/ligo-india-global-network-labs-study-gravitational-waves-8545293/

They say the acceleration is the result of a previously unknown interaction between black holes and spacetime.

Source: https://www.discovermagazine.com/the-sciences/black-holes-are-accelerating-the-expansion-of-the-universe-say-cosmologists