What's so special with Dark Energy or Dark Matter?
They are explanations for observed phenomena. Think it more like a general term for presently unknown reason that keeps the observed universe together. One could go with "unknown source of gravity / energy that keeps Universe together", but it has been named as Dark Matter/Energy since the actual mass/energy has not been observed yet, hence they remain in dark.
Or that is my understanding of the issue.
There are another explanations for those observations, like localized warping (galaxy level) of space-time. I'm not the right person to comment about the validity of those explanations. What I gathered from that stuff was that it is thought that there are more 'dense' spacetime areas and less 'dense' spacetime areas, curvature of the spacetime causing the observed phenomena in denser areas. This isn't mainstream explanation, though.
Mika