Corvidae wrote:I was hoping you'd pop up! I wasn't aware that dark matter was considered "regular matter" at all. Also, I was more referring to "dark energy," which according to current book I'm reading (circa 2006), is believed to be 70% of the matter/energy in the universe?
I gave about the simplest rebuttal I could, but yeah, there are some distinctions between dark matter and the zero-point energy people refer to as dark energy. There are also probably distinctions between "normal" matter (by which I mean composed of protons, neutrons, and electrons) such as the Earth and what makes up the rest of the dark matter in the universe. A lot of the quantity of dark matter astronomers think exists is probably neutrinos and other particles, some of which we may not have discovered yet. However, we can't detect planets orbiting other stars by a direct electromagnetic signature, so they - and also the Earth - are "dark matter" in that sense. The way I believe most astronomers think about it is that "dark matter" is an umbrella term encompassing all the mass that must be there to account for the observed orbital speeds of stars about galactic centers, and nonluminous "normal" matter is just one component of that. They also sometimes say "dark matter" when they mean all the matter in that first set other than "normal" matter. At some point, it's just a matter of lexigraphy rather than a real physical distinction. The point of my first post was that it's not just the fact that some dark matter has to be around to "make our equations work" that makes scientists hypothesize its existence. We can directly observe some of it (the Earth), and indirectly observe other parts of it (neutrinos).
Dark energy is a different physical concept, but I can make some defense of that hypothesis, too. The basic concept to have straight is Doppler shift, like how a train whistle or the drone of an airplane engine changes in tone depending upon if it's travelling toward you or away from you. That's a pretty fundamental tidbit of physics; it's used all the time in science and engineering, and there are so many things it explains completely and correctly that it's difficult to call scientists arrogant for assuming the concept is right. Astronomers use the Doppler shift of light emitted from distant stars to determine how quickly they move toward or away from us. One thing they have observed from measuring this rate from many stars is that the universe seems to be expanding at an accelerating rate. Everything to this point is backed by really solid basic science. Now, we'll get into people's hypotheses. Astronomers have been trying to figure out for decades why they see accelerating expansion. This is where "dark energy" comes in. Assuming that there is some energy uniformly spread throughout all space is one somewhat simple way of producing this accelerating rate of expansion. Although astronomy isn't my specialty, I don't believe I've ever heard a professional astronomer claim that any particular dark energy model has been proven correct. It's not arrogance that's driving them to think about it; it's simply an effort to understand and explain what we can measure and see.