re: Dark matter and dark energy.

You're right. We don't have a clue as to what dark matter is. We can only see its effects. That's why I mentioned that the term "dark matter" is only a placeholder name. No one should think that scientists are assigning any other properties to it, whatever it is.

The reason dark matter and dark energy are being used comes from basic physics. Einstein's equation, e=mc2, tells us that everything is either matter or energy depending upon its current state. Matter in motion is energy so energy not in motion is matter. Both exert gravity. Matter tends to be stationary so the term dark matter is used to designate a possible entity that's affecting galaxies and light.

Of course, the common term that's involved in any of this speculation is gravity. Despite hundreds of years of research, thought experiments and math by humanity's greatest minds, we still don't know what gravity is. We are extremely familiar with its effects and can predict it with great accuracy, all without knowing it's composition or cause.

Is it a force or an effect? Is it truly one of four or five basic forces that originated when the Universe was born? Or is it the effect of space upon mass?

Is it a pull or a push? Are there really gravitons that carry a force that binds mass together or does space actually push mass together? We just don't know.

Quantum physics may answer this and other mesmerizing questions. It's fun following our human progress towards solutions.