Just to clarify one point, while there are things in science that are theorized to exist but have not been seen yet (Such as the Higgs particle and Dark Energy) they are not unobservable, just we have not manged to do it yet.
One problem is that the setup to make these observations tends to be HUGE and cost huge amounts of money, so it takes time. There are people actively trying to figure out Dark Energy, they just don’t have enough data yet, and the current generation of telescopes are not quite powerful enough to get the data that is required.