Are the "dark matter" and "dark energy" misinterpretaions of observation?
Since we know of the "dark matter" and "dark energy" only by their gravitational effects, might it be that the problem is with our understanding of gravity itself?
Could gravity decrease more slowly at interstellar distances than current theory predicts thereby holding galaxies and clusters together, and then become negative at inter-cluster distances so as to cause the accelerating expansion of the Universe?
Just speculation.