UFOs are simply things that not readily identified. They are not 'alien' spaceships. Nor do they defy gravity and there are many phenomena that do not make any noise.
It doesn't, nothing can defy gravity.
No
The plural of defy is defies. As in "this defies the laws of gravity".
The chemical that is called defying gravity.
Ultimately, no.
No they don't.
adhesion.
They cannot.
The water seemed to defy gravity.
No, they don't.
No.
nothing