Are you thinking of something like neutronium? This is the (hypothetical) matter formed when you compress the electrons into the protons to make neutrons, then pack the neutrons tightly together. If so, then the density is $4 \times 10^{17}$ kg/m$^3$.

However you should note that even neutronium isn't pure matter, because neutrons are made up from quarks and there is free space inside the neutron between the quarks. There have been suggestions that if you compress neutronium you could collapse if further to make strange matter with a density that is about 100 times higher still. However this is currently only speculative.

Are you thinking of something like neutronium? This is the (hypothetical) matter formed when you compress the electrons into the protons to make neutrons, then pack the neutrons tightly together. If so, then the density is $4 \times 10^{17}$ kg/m$^3$.

However you should note that even neutronium isn't pure matter, because neutrons are made up from quarks and there is free space inside the neutron between the quarks. There have been suggestions that if you compress neutronium you could collapse if further to make strange matter with a density that is about 100 times higher still. However this is currently only speculative.

By coincidence a paper on this subject has just appeared on the Arxiv: Properties of High-Density Matter in Neutron Stars