You should note that the existence of dark energy is deduced indirectly from motion of celestial bodies under the assumption that the standard general-relativistic theory of gravity holds. Also, and this is a more general point, educated specialists in any field - not only physics - aren't stupid and really know their field. Therefore, if you, as a non-physicist, can come up with a simple theory of gravity that is not believed by the physicists, the theory is almost certainly false - somebody certainly had the same idea long ago and there were reasons to reject it. Keeping this in mind can save you a lot of time and futile effort.

For reasons why your theory is false please follow the link in Mitchell Porter's comment.