What is the densest thing in the universe?
A black hole
A black hole is said to be a singularity. That means it's no space between the mass and it is pure mass. No atoms, no quarks, just pure mass.
Arguably the densest thing in the universe is a neutron star.
A neutron star is the collapsed core of a star. They are formed when the core pressure exceeds the Chandrasekhar limit of about 1.4 solar masses. The electron degeneracy pressure is exceeded which forces protons and electrons to combine to form neutrons. Neutron stars have a density of
The discussion of density gets more complex when considering black holes. Stellar black holes have very high densities. General Relativity predicts a singularity, which is a point of infinite density, inside the black hole. When an infinity occurs in physics it suggests that the theory is wrong in those conditions. We can't be certain what is inside a black hole at our current level of understanding.
The density of a black hole is inversely proportional to its mass. A supermassive black hole, such as the one at the centre of our galaxy, has a density less than that of water!