If I have Darkrai-GX in my discard pile but no Darkness Energy, can I use its "Restoration" Ability?
Yes, Restoration only requires that Darkrai-GX is in the discard pile (and that you have space on your Bench for it).
Source: Burning Shadows FAQ (2017-07-20), TPCi Rules Team (2017-08-03)

Relevant cards

Note: All applicable cards may not be listed below.
SM3_139
SM3_158
SM3_88
SM3_88a
SMA_SV70
SMA_SV70