Air carrier First Officer reported that while exiting Runway 27R at PHL; taxiways K5 and K4 are confusing due to poor lighting.

Narrative

I was the PM and the CA was PF. We were landing late at night in PHL on runway 27R; with moderate rain; shortly after thunderstorms in the area. We had briefed exiting the runway at K5 and taxiing eastbound on taxiway Kilo to our gate. The landing was uneventful; and we began to exit the runway on K5 as briefed. The K5 taxiway exit on 27R (as well as K4) is very confusing unless you recognize and brief it beforehand. The high speed taxiway splits shortly after departing the runway; with the west K5 going straight ahead and the east K5 splitting into a 180 degree turn to join K eastbound. Between these two K5 taxiways is an unlit paved area with signage that is not illuminated from the runway side; nor are there taxiway edge lights. This is only discernible on the 10-9 pages or the AMM (Airport Moving Map). The high speed K5 taxi line/lead off lights intersect the K taxiway at a 90 degree angle and the lead off lights do not gradually join the K taxiway. The low speed K5 has lead off lights that do a 180 turn to the right and joins K eastbound. As the CA exited the runway; our attention was on an Aircraft Y on Kilo that we were instructed to follow. The CA split the difference" between the two K5 taxiways; as the low speed taxiway would have been unsafe at our taxi speed and the high speed K5 did not have an illuminated lead off path that would have properly joined K in a typical manner. When I returned my scan to the taxi path of the aircraft; I recognized something was wrong; and immediately thereafter saw two unlit signs in the taxi light of the aircraft and commanded "stop". The CA immediately stopped the aircraft; I pointed out the hazards; and the CA navigated us around the non-taxi area without incident. We never departed the approved taxi area (the non-taxi area was a paved surface); but we did come very close to hitting multiple signs and other lights. After debriefing and doing a thorough post-flight walk around; we decided out of an abundance of caution to inform the Chief Pilot and have PHL airfield ops and maintenance inspect the taxiway exit area and the aircraft to be 100% sure we did not hit anything or depart the taxiway."