Update 08/29/18: The cube still seems to be empty but is now surrounded by a low-gravity dome, which appears to have the added effect of slowly increasing the shields of anyone who walks inside it. In addition, the base of the cube now glows pink. There’s still no official word from Epic, but you can see the dome in this tweet:
The cube is now surrounded by a dome of low gravity! #Fortnite pic.twitter.com/dNHrxcVmAn
— Fortnite Battle Royale News (@FortniteBR) August 28, 2018
Source: IGN.com The Fortnite Cube Now Has a Low-Gravity Dome Around It