That's the million dollar question isn't it.
The answer?
True earth history, biblical history would reveal to you that yes, they
existed in physical form.
Fallen angels in the flesh mated with human woman thousands of years
ago and created the nephilum.
Human form, human bodies, satanic spirits.
When these fallen angel children died their spirits remained on earth
and are now what is known as evil spirits.
This infestation of evil polluted the human population and is ultimately
why God (in large part) flooded the world (according to careful research).
In order to delete these villans from the face of earth.
Their spirits remained.
Initially they regained a foot hold in the world again through human
possesion. Willing hosts whom they deceived.
The original fallen angels still exist in physical form. God burried some
under mountains and essentially chained the rest up in dark secret
places.
Their evil human followers have searched for them for a millenia.
The question is, did they ever find them?
Are they now using their seed to produce more of THEM in secret places
waiting for their time.
Will they present themselve to the world again, but this pretending to be
aliens from another planet.
They answer, is yes.
How exactly did they become flesh again? That story is for another time.
It is only important to know that some present themselves through
willing possesion and others present themselves as actual humans
in the flesh. You only true defense is God. The Holy Spirit.
Just like in the movie the matrix, if you're neighbor is not protected
they can be used, possessed.
So, anyone (accept believers) can be used as hosts and they can
move from one person to another (just like the matrix movie - accept
these are no computer programs).
So beware, prepare you're defense withiin the word of God before you
dig to deeply into this area, for that is truly the only defense you have.