Dark matter's effects have been observed, mostly in galaxies, but dark matter itself has never been detected. We don't know what it is.
etc
Perhaps electricity and the history of the development of electrical theory would make more sense to him. Even though his mind might reflexively shut down at the first mention of Ohm's Law (since he might misunderstand that too as being Eastern mysticism).
We had long observed electricity in the forms of lightning and as static electricity, though without making the necessary connection between the two. Experiments with static electricity led to the idea that there were two different kinds of electricity: one produced by rubbing amber with fur and the other by rubbing glass with silk. Even though they didn't know what static electricity was nor even had any kind of good guess (ie, they though it was two different kinds of "subtle fluid" or whatever would serve as carriers of electrical charge, which we later learned is wrong), they still made excellent observations of how it behaved and developed techniques and equipment (eg, Leyden jars) to work with it. IOW, not knowing what it was did not stop them from learning how it behaved and how to work with it.
I had been taught that it was Benjamin Franklin who figured out that those "two different kinds of subtle fluids" were actually greater and lesser amounts of the same charge carrier and furthermore formed the hypothesis that lightning was actually static electricity. Regardless of who actually came up with the idea, it was Franklin who conducted his famous kite experiment in which he collected the electric charge in a thunderstorm into a Leyden jar (which serves as a capacity, AKA "condenser", to store an electrical charge) in order to demonstrate that it behaves just like static electricity.
Even though he now knew a lot more about that charge carrier, Franklin still didn't know what the charge carrier was, nor which "kind" of static electricity was due to more charge as opposed to less charge. So he mentally flipped a coin and decided that the glass/silk static electricity contained more charge carriers -- hence declaring its charge to be positive (+) -- and the amber/fur type lacked charge carriers -- hence declaring its charge to be negarive (-). It turns out that he had gotten it wrong, but that wasn't discovered until a century later.
Franklin's research, along with Volta's invention of the battery, led to the development of electrical theory filled with mathematical analyses of electrical circuits, including Ohm's Law, which electrical engineers and technicians still learn and apply in their work. That also led to the development and invention of electrical devices (eg, the telegraph) which not only worked but they still work
even though the inventors still did not know what that mysterious charge carrier was. Obviously, not knowing
what it was did not keep us from learning
how it worked and how to use it.
It was one of those devices, the cathode ray tube (CRT), which led to the identification of that mysterious charge carrier, the electron, and the embarrassing fact that its charge is
negative.
In a similar manner, we can observe the effects of
something and measure them and work out a theory of how that
something works. Furthermore, we can make good use of that knowledge even though we do not know what that
something is. One type of that
something that has one kind of effect we call "dark matter" and another
something that has another kind of effect we call "dark energy." And even though we don't know what they are, that does not keep us from studying them and figuring out how they work.
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