Guys - I need some help here. I am just an interested layman, so i dont have deep understanding of the math involved in gravitational theory, but I remain quite confused regarding the concept of dark matter and dark energy. I understand that this is a result of large-scale gravitational observations, wherein large bodies of mass (galaxies, clusters, etc) do not behave exactly as predicted by GR, but i have great trouble imagining that there is some magical entity of mass/energy that is completely undetectable. from my limited understanding, it appears that a slight modification of GR, for example an adjustment of the gravitational constant, might be an alternate explanation of the observations - is that correct, or can such adjustment not truly explain what we are seeing? if not, how do you reconcile the idea that dark matter and dark energy exist without being detectable, and in apparent conflict with the very successful standard model? thanks.