Why if everything is flying apart do galaxies stay roughly the same size.
Galaxies coalesced out of the uneven spread of energy following the Big Bang. Once they settled into physical form and converted to matter they are then bound by gravity, and many are centred on black holes. Gravity keeps them together even while galaxies are moving away from each other.
Matter can be expressed as a quantity of energy. It just takes a lot of effort and energy to release that energy bound within the atom, hence the Atomic Bomb.
Energy is expressed in Electron Volts, which are not the same as the Volts in your household current. Generally the energy in atoms is expressed in Millions and Billions of Electron Volts, because if you multiply the mass of an atom times the speed of light squared, you're going to come up with a huge number.
The Hubble Constant or Cosmic Constant is a theoretical number for a steady-state universe neither expanding nor contracting. If the observed number is higher then the Universe will collapse, if the observed number is lower the Universe will continue to expand. Since we cannot come up with an observed number, owing to an unquantifiable amount of Dark Matter/Energy, we still can't figure out the answer to that question. but the speed of expansion should at least give us an indication that it is lower. But this is Physics, if it makes sense, it's almost certainly wrong.
Why is Gravity different? It may not be. We just haven't figured out the math to prove it does indeed mesh with the other forces Magnetic, Strong Force and Weak (nuclear) Force (or have I just confused those two). It's hard enough to wrap your head around the concept that Magnetism, the Strong Force which binds atom to atom in compounds and the Weak Force which binds Proton to Proton in a Nucleus, are all part of the same set of equations. Yes, we now know there are particles called Gluons (really) which carry the Weak Force which binds larger particals called Protons to one another. However since I don't grasp Mathematics on this level, all I can do is nod with as much comprehension as a bobble headed dog in the back of a low-rider.
No matter what I gather about "dark matter" it will reel in something about the state of current physics. The tangle of physics of not only the cosmic but also the quantum variety. They become linked one way or another. I haven't really gotten anything that pin-points what dark matter is, other than theory. I understand this is still an unknown but overall it seems to grossed over and/or humped into the current state of physics. It's hard enough trying to figure things out and then separate one theory from another that has nothing to do with the other. BUT! it's all linked in some grand scheme of some sort. While, I have always stated here and elsewhere I probably could never understand the math involved, it sure seems like many of the models are purely hypothetical in nature or merely a step-up from present understood "standard model theory". Further, while I query about about dark matter, soon enough dark energy becomes tangled in as well. They become more in focus as the "dark subject" as you can't mention one w/o the other. As i have stated in the past,, the best proof has been on the "galactic scale" of things. It takes that much to show something. In other words, the universe is the lab and no amount of high-energy collisions at CERN, Livermore or any other collider is going to prove it. While the Boson-Higgs particle and all it implies is the result for now, what does that do for dark matter. No link that I can think of. Maybe, I missed it but it shows mass(how it provides) and maybe can be attributed to neutrinos and the like which are mysterious particles too. It's not like a jigsaw puzzle and you try to join the pieces, it's more like thinking there is a jigsaw puzzle and you can't find it but theorize what it may look like while you look.
I just finished another book, called: Scientific American, Cutting Edge Science, Extreme Physics. Somewhat impressive but lacking in anything for dark matter. Maybe a few lines on dark matter and energy but nothing that smacks of a clue. However, a link is provided for further inquiry on physics:
http://www.physicscentral.com/explore/action/camera1.cfm <---decent article and more links
enjoy -----Willy still looking (o,o)
E = MC2 uses that as basis....
? - If matter is less energy, how does it not lose matter as it just "is" while its active or remain active particle?
? - If energy is not matter, when it loses energy does it become matter?
? - Vacuum energy or dark energy really the basis for the cosmic constant? And if so, why do galaxies remain as a whole yet still fly away from each other in the accelerating universe? Shouldn't that imply that everything fly apart.
? - Why is gravity so different from the other 4 known forces?
Answer back, and win nothing because no one know anyways, it's all theory. -----Willy