Correct me if I'm wrong, but I find it fascinating how dark matter and dark energy are commonly brought up together, yet describe very different observations for which we have quite different degrees of certainty.

With dark matter, we are at least relatively sure that it's some form of matter. It might be something we already know but can't see out there, or more likely, it's a new form of matter that we haven't seen before. Theories like modified Newtonian dynamics that attempt to explain away the evidence for dark matter by adjusting the laws of physics seem to fall flat so far.

Meanwhile, dark energy could be anything. It could be some unknown particle field, could simply be a cosmological constant or maybe our understanding of cosmology itself is fundamentally wrong.

The answer to that question is: ‘Empty Space’.
Although we know not even that is empty…

Of course, there is the occasional lump of ‘stuff’ scattered around in that space too - which we call ‘matter’.