Things are actually kinda empty that's why there is gravity, it's a result of pressure effect.

So Earth is kinda empty because its particles are kinda crystals that push things away. But thanks to the pressure of vacuum there is this effect. Matter goes in direction what is empty.

So the "space" outside Earth is almost full and getting part of the hurricane effect cause by the particles of Earth.

Particles of Earth have fields that push things away because its spinning or pulsing. Then when things get pushed away there is a pressure that pull things close again.

So that's what happens with a galaxy, it's spinning and pushing things away but there is an effect that calls matter into its direction thanks to the effect of vacuum pressure.

So a black hole has lots of matter, which are things that has an empty or almost emprty nucleous, that's why gravity exists.