The darkness rifts are the way that players go back to orbit in Destiny 2. They appear very often, and they cannot be closed with melee attacks or grenades. Instead, you have to clear them out by shooting one of three weapons at a time repeatedly: your sword, primary weapon (rifle), or Special Weapon (pistol). If a rift is too large for your current loadouts ammunition capacity, then use an alternate gun until it can close on its own again.