Let's focus on black holes now. When astronomers speak about them, they often make an unintentional impression that they are some kind solid objects. They are not. A black hole is a spacetime singularity that is enclosed by an event horizon. Both things are quite weird, but none of them is anything solid. read more
So if you consider the singularity to be the black hole and consider the event horizon to be merely an effect rather than an intrinsic part of the black hole, then indeed you may consider a black hole to be solid. If however you include the event horizon, then black holes will contain a lot of space. read more
A Black hole is not a single thing. A good deal of what defines a black hole is constructs of spacetime that exist because of the mass. There's an event horizon, which is part of that spacetime construct. However, material can exist throughout the black hole in different forms. read more
Yes, black holes are solid matter. The terminology of "Black Hole" is considered somewhat misguiding. This term refers to the fact they are so massive and condensed they won't let even light escape. read more