I was just wondering because Galaxies seem to act like solids. Combine that much dark matter mass and regular matter mass all in the same place and perhaps it acts like a solid.
The states of matter such as solids, liquids, or gases are as a result of the electromagnetic
force, rather than gravity. Namely objects are considered solid when the molecules form up into into highly ordered grids called a crystal, much like a military marching band where individuals move in unison. In a liquid, the electromagnetic force is much less, and the molecules don't really move around in lock step with each other, however they do want to cling
together in an overall grouping, such as drops of water; this is an effect called surface tension
, which is an electromagnetic effect. In a gas, there's even less structure, and really one can say that the electromagnetic force is exerting nearly no force at all on the molecules of a gas, individual molecules of gas would fly randomly off to all corners of space if it weren't for a secondary force like gravity holding them together.
Most would consider a galaxy is closer to the gas model than the solid model. But some may argue that a galaxy is closer to the liquid model than the gas model. Individual molecules of a galaxy (stars) like to tend cling together in an overall grouping, much like water droplets. Of course at these scales, it's not the electromagnetic force that's responsible for holding them together, but gravity. So it's possible that there's a gravity-based surface tension effect on the stars of a galaxy.
There was an alternative theory to Dark Matter
, Dark Energy
, and MOND
, called Dark Fluid
, which modeled the universe like a fluid. Specifically it treated the universe like a weather system on Earth, where you have calm areas and hurricanes (galaxies) spinning up and their spin actually binds them together more tightly.