I am not sure it is physical to speak of black holes as having a density, since unlike neutron stars, black holes are not really composed of stuff, i.e. matter with mass. The event horizon is not really a thing, so much as a boundary between our spacetime and that of a singularity, and while we can assign a "density" score to its contents, it is not constant but an inverse-square of said contents' mass (which is why, as qqvtn said, black holes with less than about 5.5 solar masses should be denser than most neutron stars). That said, even neutron stars vary a bit in their density, depending on their size (though not nearly as much as black holes -- within a single order of magnitude), while at standard temperature and pressure, neutronium should be a gas even less dense than molecular hydrogen. It is the degenerate neutronium that forms under extreme gravity and pressure that most people think of as the ultimate density measure.
tl;dr Neutron stars are as dense as it gets with normal matter. If you allow for spacetime shenanigans, stellar-mass black holes under 10^28 metric tons are what you want. Or you could always call up your mom. ;P
last edited at May 10, 2022 4:14AM
