This is because "dark matter" and "dark energy" have never been directly observed, they are inferred theoretically as explanations to as yet unexplained astronomical observations.
I liken it to the theory of God, minus the vanity and parables.
Why can the scientific community not explain what "dark matter" or "dark energy" are?
As "The Z" stated.
Also, just because scientists can prove something exists, doesn't mean they fully understand it. Such as the research that shows quantum linkage between particles can be proven and artificially created - but they don't know how, why, or even if it occurs naturally.