I remember actually doing the math to figure out the bloody thing's mass based on formulas for neutron star core material density. Sca-ry.
If it were possible for neutronium to exist. (Nuclear chemistry says no.) Still, since you're talking fiction, I'll grant your neutronium. The builders of the DM stole the neutronium bricks out of the gateway to a wormhole highway.
I didn't make up that it was neutronium, that was pre-set. I just used a 1990 science text about neutron star material density to see what the results would be if someone would figure out how to make something stable out of such material.