We know that black hole is infinitely densed.
More exactly: The theory of general relativity predicts that the center of a black hole is infinitely dense. This theory predicts very well everything about gravity (including black holes). But it does not account for elementary particles in any way. Hence we cannot trust this theory on the particle level.
On the other hand, we have the standard model of particle physics. This theory explains very well everything about elementary particles and their interactions (electromagnetic, strong nuclear, and weak nuclear). But it does not account for gravity in any way. Hence we cannot trust this theory in strong gravitational fields.
Until now there is no confirmed theory of quantum gravity which would explain both gravity and elementary particles. There are several candidate theories, but all of them are still speculative, and none of them is confirmed by experimental facts. Hence we don't know what is at the center of a black hole on the particle level. We don't even know if our concept of elementary particles still makes sense there. May be particles are only valid as an approximation in weak gravitational fields.