Gravitational waves are disturbances in the curvature of space-time caused by the motions of matter. Though gravitational waves pass straight through matter, their strength weakens as the distance from the original source increases. Although many physicists doubted the existence of gravitational waves, physical evidence was presented when American researchers observed a binary pulsar system that was thought to consist of two neutron stars orbiting each other closely and rapidly. Radio pulses from one of the stars showed that its orbital period was decreasing. In other words, the stars were spiraling toward each other, and by the exact amount predicted if the system were losing energy by radiating gravity waves.
Most black holes tend to be in a consistent spinning motion as a result of the gravitational waves.
The matter keeps within the event horizon until it has spun into the center where it is concentrated within the core adding to the mass. Such spinning black holes are known as Kerr black holes. Time runs slower where gravity is stronger. If we look at something next to a black hole, it appears to be in slow motion, and it is. The further into the hole you get, the slower time is running.