Quick Question: Optistruct Tutorial Units??

Hello,

I'm currently running through tutorial OS-3300: Using the lattice optimization process in optistruct. I have run the studies with various porosity and want to compare the mass of the interations. However, I am struggling to understand the unit system used as Density for the material is at 7.9*10^-9? Does anyone know what uniots have been used?

Thanks in advance,

Tom