Riddles In the Dark - It cannot be seen, cannot be felt...

Riddles in the Dark - It cannot be seen, cannot be felt... (Found at Astor Place station; downtown 6 train)

Riddles in the Dark - It cannot be seen, cannot be felt... (Found at 23rd street station; uptown 1 train)

Riddles in the Dark - It cannot be seen, cannot be felt... (Found at 7th Ave station Brooklyn; Manhattan bound Q & B trains)

Riddles in the Dark - It cannot be seen, cannot be felt... (Found at 50th street station; downtown 1 train)

 Riddles in the Dark - It cannot be seen, cannot be felt... (Found at Chambers street station; uptown 1, 2 & 3 trains)

 Riddles in the Dark - It cannot be seen, cannot be felt... (Found at 50th street station; uptown 1 train)

 Riddles in the Dark - It cannot be seen, cannot be felt... (Found at 28th street station; downtown 1 train)

 Riddles in the Dark - It cannot be seen, cannot be felt... (Found at Astor Place station; uptown 6 train)

Riddles in the Dark - It cannot be seen, cannot be felt... (2015)

Answer: The dark.