You know what else is hard?
A black hole compresses mass so much that its density becomes extreme beyond normal matter physics. At the center is the “singularity” idea, where current physics breaks down and density may become effectively infinite.
Number 2 is:Neutron Star matter
Neutron stars are insanely dense:
A sugar-cube-sized amount of neutron star matter would weigh about a billion tons on Earth.
For comparison:
Osmium (densest natural metal on Earth): ~22.6 g/cm³
Earth’s core: ~13 g/cm³
Neutron star matter: around 100 trillion g/cm³
So:
1. Black holes → densest known objects overall.
2. Neutron star matter → densest known physical matter outside black holes.