Update 18/29/18 Part 2: The cube has been leaving behind mysterious runes in its wake. Watch the video below for more.
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: