Some scientists have suggested that they might be solid objects but they are not fully in our dimension, darting in and out between ours and theirs when we have seen them. This would make them quite similar to holograms except they would be detectable by radar and other imaging technology (which they are)...and give them the appearance of violating our physics in their movements when they actually do not.
That is also very reasonable an explanation. It would also jive well with the Israeli retired Space Force leader who told us that they are in communication with us, but that they will not want it generally revealed until we (the common masses) understand what space REALLY is, and what spacecraft REALLY are.