I want you to get the picture here. This awesome being of power that was so great that men (and I have to think that these were ordinarily strong, valiant men) were so frightened at something they could not see that they got up and ran, and yet something withstood this great being to his face and kept him from getting to Daniel for three full weeks.
Can you even begin to imagine the titanic struggle that went on between (we will say) Gabriel and whomever this other being was? There must have been an awesome wrestling match like you and I have never been witness to.
I used the term wrestling match because I do not know what it was. But whatever it was that withheld Gabriel from getting there must have been awfully powerful. I want us also to see that this great struggle was going on without Daniel even being aware of it. Somehow or another a malignant demon tried to thwart Daniel's prayer from being answered. But it was answered and it was answered because, in verse 13, "Michael, one of the chief princes, came to help me, for I had been left alone there with the kings of Persia."
It was not until two of them ganged up on this other—two great, mighty archangels to subdue this other, whatever it was. I think the logical conclusion would almost have to be that the king of Persia that is mentioned here was none other than Satan.