The estimate is that there are 400 billion star systems in the galaxy, and that's the assumption that FDev seems to use when they mention percentages. Less than 0.02% of that is known to EDSM, and I think no more than about 0.05% (or maybe 0.06%) has been discovered overall in-game. I don't remember the exact number for the latter, and of course it changes over time. In any case, it's below a tenth of a percent. 