Gravity can be detected by its effects, even if there's no particle that propagates it. We've been able to detect not just the strength of gravity (by weight) but also its propagation through space (see LIGO).
>Has a mind dependent of a brain been observed?
Um, obviously? What distinguishes a person from a braindead patient is that one of the two has a mind. Are you saying that the fact that you talk to a person and have them recall facts is not enough to say that they have a mind? Also, to say that a mind has never been observed either dependent or independent of a brain would mean that the idea of mind has no basis in reality at all.
>At most it can examine its interaction with matter
Well, ultimately yes, because any instrument we are able to construct will necessarily have to be made of matter. The upside of this is that if there's a phenomenon that does not interact with matter in any way whatsover, even indirectly, then that phenomenon cannot possibly have any relevance to human life. If minds are an inherent physical phenomenon that can interact with brains, then we should be able to construct a device that's able to interact with them. If this is not possible even in principle, then I don't see how minds can be said to be fundamental. What aspect of reality informs that belief of yours?