"Dark Matter" is a place holder for a behavior we can see, model, predict and test the effect of, yet cannot explain the why we see those things. Hell, we are still not 100% on what exactly gravity is. We can model it, make predictions, use it to send spacecraft across the solar system and all, but it was only in the last few years that the Higgs Field was discovered right where the predictions said it would be. New observations and data came in and the unknown became the known.

"Dark Energy" same thing. SOMETHING is pushing the galaxy clusters apart and is expanding the size of the observable universe. We can observe its impact, make predictions, generate mathematical models on what it is doing, but are still in the "dark" about what is actually the root cause of the effect.

The interesting and exciting thing is that the stuff we know with a very high degree of confidence is only 4% of the universe. 2/3 of all matter is a placeholder in an equation! The next 50 years of discovery are going to be mind blowing if we survive long enough to figure it all out.