Black holes are not solid objects in the traditional sense. They are best described as regions of extremely compacted mass or energy that result from the collapse of massive stars. The concept of solidity as we understand it does not apply to black holes.
According to general relativity, black holes are formed when a massive star undergoes gravitational collapse, causing its core to become incredibly dense. This density results in a gravitational field that is so intense that nothing, not even light, can escape its gravitational pull within a certain distance from the center. This region is called the event horizon.
The central part of a black hole, often referred to as the singularity, is a point of infinite density where the laws of physics as we know them break down. It is thought to be a location of immense gravitational forces, but our current understanding is limited when it comes to describing the singularity accurately. The singularity is hidden within the event horizon and is not directly observable.
Therefore, while black holes have a defined boundary (the event horizon), their interior and the singularity are not understood fully within the framework of our current scientific understanding.