Question: Are the "dark" theories Eg matter, energy, flow real measurable forces or are they a simply a way to name the yet unexplained?

Sara Imari Walker answered on 15 Jun 2011:

Ah! One of my favorite topics!!! =)

I just answered a question on dark matter for joshhook. Here is what I wrote:
We don’t know what dark matter is, but we do have some clues. The term dark matter arose because it appears that most galaxies have extra mass that doesn’t interact electromagnetically (i.e. we can’t see it since it doesn’t emit light like stars). The existence of this dark matter was first surmised by looking at galatic rotation curves – as you move away from the center of galaxies you should see the orbital velocity (their speed circling the galatic center) of stars slowing down as the density of stars drops off due to less gravitational attraction. Instead you observe a constant oribital speed – even to the far edges of galaxies -as you move away from the galatic center -indicating that there is some unseen matter there that interacts gravitationally! So we don’t know what dark matter is but we know it interacts via gravity. We do have a few candidates for dark matter though. One is brown dwarfs – those are stars that didn’t quite make it to Hydrogen burning so they are effectively dark dead stars. Another is small black holes. My personal favorite is WIMPS (weakly interacting massive particles) a hypothesis that claims there are new fundamental particles that we haven’t detected yet (fundamental being something like an electron or a quark). WIMPS would have to have some very special properties to be dark matter – imagine a particle that doesn’t interact with light! I did a project once on axions – those are pretty cool little particles – I hope they exist!! But there are ALL kinds of particle candidates ranging from supersymmetric (like a mirror particle with large mass and some opposite properties for all known particles) of even sterile neutrinos (these are really heavy neutrinos). Pretty wild!!

So yes, dark matter is very real and may possibly be detected with particle detectors we have out there now. Dark energy is, as Drew sad, on less solid a footing. We do have extremely good evidence that the expansion of the universe is accelerating. How do we know? Well we can observe things like supernova very very far away. What we notice is that these objects are moving away from us faster and faster the farther out we go. That means the cosmic expansion rate is accelerating. We do have some ideas why. It does have to do with balancing equations – what we are trying to balance is vacuum energy. That is the energy of empty space. Turns out the universe is probably dominated by vacuum energy!! The trouble with dark energy though is that the numbers don’t balance out. So, we have a lot more work to do on this one!! Maybe someone like you will solve the mystery for us someday =)