Magpie
Well, let's make a distinction between "the supernatural" and "supernatural claims", shall we?
Firstly, there's simply no evidence that there's any such thing as the supernatural. Indeed, if there was evidence for such a thing then there would be nothing "super" about it...it would simply be natural. The universe is all known things and natural laws are all known processes.
Science cannot be expected to go looking for something outside all known things because, you see, things have to be known in order for us to be able to go and look for things there...otherwise, how would we know where to look? This is why science does not investigate "the supernatural". It can't.
What it can investigate is "supernatural claims" where people like you claim that some event is real and true and the only reason that scientists can't detect it is because it's "outside of all known things" aka "supernatural".
But here's where you are going wrong....the event described and person experiencing it are not "supernatural"...they are very much natural. They exist within the known universe and, as such, are subject to the same scientific tests as anything else. Because, and here's the thing, even an event that has "supernatural" origins MUST have a natural component in order to make itself known within our universe.
A "universal life force" that can relieve lumbago cannot be anything other than a natural phenomenon with measurable, natural effects.
Science can investigate anything...and it does. The only people who raise the "supernatural" as an explanation are people, like you, who hate the fact that your personal beliefs are not supported by evidence. So you lift up the escape hatch, climb out and declare that, "Oh well, maybe there's a supernatural explanation" ignoring the fact that it's just been proved to you that no explanation is needed at all.
Reikki doesn't work. It doesn't cure anyone of anything, There can be no natural or "supernatural" explanation for a phenomenon that doesn't exist because....it doesn't exist.
Science does not make the distinction you are making between natural & supernatural claims. It just deals with the claims. "Supernatural" is an excuse, and rather a childish one at that.