Comment by sbierwagen

It can't. The galaxy is assumed to be roughly symmetrical, and they fill in the missing data with what we can see on our side of the galaxy. It's "best" in the sense that it's the most accurate fiction, I suppose.

Gaia is good to about 13,000 light years: https://en.wikipedia.org/wiki/File:Galaxymap.com,_map_12000_...

The Milky Way is maybe 100,000 light years in diameter. So we're only getting good distance readings on a small fraction, and nothing behind the central bulge of our galaxy. The first won't improve until we send an astrometry telescope way outside the orbit of the Earth, for better baselines, and the second is going to need a telescope sent 10,000 light years out of the galactic ecliptic.