I say density, though Elite Dangerous puts a spin on how large the map should be.

In Elite Dangerous, most of the galaxy is unexplored. The Bubble (human inhabited area) is fully explored, which steadily dwindles as you go to about 1k ly outside the Bubble. Out there, you’re basically on your own.

When you explore and map unexplored areas, you actually get some money depending on the quality of your finds. If you find some Earth-like planets, for instance, you can get a lot of money from exploring. There is also an inexhaustible supply of systems to explore, so there’s no need to worry about running out.