I'm reading a NASA paper about the Great Observatories (for grade schoolers) in which is says that "gamma rays can only be detected when they interact with matter." Surely we can only detect anything when it interacts with the matter of our detecting instrument, but some helpful people at Rutgers explain that with gamma rays, what we actually record is the interaction of these rays with other matter.
What makes gamma rays (as opposed to other kinds of electromagnetic radiation) so special that they can't be detected directly, like visible light? Are they directly undetectable in principle, or is it just that we haven't built any instruments that can deal with that high an energy level, or what?
EDIT TAKING COMMENTS INTO ACCOUNT: The things I've been reading make it sound like visible light just sets off a detector, whereas to detect a gamma ray I'd have to set up two devices--one to interact with the gamma ray and one to detect the visible light (or whatever) from that interaction. The first is what I'm calling "direct detection," even though there's a chain of things happening in my detector. Is this a distinction without a difference? I figured there was something different about gamma rays or they wouldn't have bothered mentioning it.