The quote you're looking for is "Any sufficiently advanced technology is indistinguishable from magic." But you have to remember that magic, too, has rules.

All I'm saying is that I think the explanation of "it's nanotechnology, I don't have to explain it" is kind of boring. But you'll run across any complications that can come from this as you continue to flesh out your world. My only suggestion is that when you start placing buildings and such it might be a good idea to place some ancient ruins that fell off the shop or that were abandoned when they were no longer useful. If your players should find a "magical fire lance" (laser rifle) within them I think they'd enjoy it.