Tom Clancys The Division has an special area called Dark Zone which is a much harsher and brutal version of the main game. This zone also carries a different rank and experience points system compared to the main game. If you want to farm experience points, credit or loot in Dark Zone, you can follow the guide listed below (courtesy of reddit). The route covers Dark Zone 01 and Dark Zone 02 only.

[W 31st St > Ext. Point(mobs, sometimes a boss) > E 33 St. > Basketball court(mobs, sometimes a boss) > Underground (guaranteed boss mob) > basketball court > Sports Centre (boss mob) > Ext. Pt. (no boss, but your loot should be full) > Corner of 6th Ave and 38th St. > Safehouse (for ammo, if needed) > Madison St. > W 31st St. (boss mob) > Repeat.

You can also check out the video detailing the route below. The user who shared this route recommends that the player should reach the max level cap, which is 30, before attempting this route.