When I first started learning Reiki, many years ago, I was often asked "is it real or is it all in the mind?"
My answer to that was, initially, "I don't know"
After many years of practicing Usui Ryoho Reiki I still do not have a definitive answer and doubt very much if anyone ever will!
What I will say is this.
When I follow my strict "rituals" prior to conducting either Hands On or Distance Healing, I do experience a great deal of Energy flowing through my body and that the majority of those who requested healing experienced the same.
So, to answer the question, for me, Yes, Reiki is "real".