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I think you mean they are gravitationally attracted to each other. Galaxies appear to have relative motion away from each other in spite of this attraction so the attraction alone doesn't determine the direction of movement. A rocket being launched is still gravitationally attracted to Earth, but it's not moving toward Earth.
Objects in space move towards other objects in space.
Galaxies are in a sense "being flung out" from each other. Stars within them are not being flung out from the galaxies as the rotational speeds would suggest if there's no dark matter. So your latter statement seems to answer your own question why the metric expansion of space offers no explanation on the rotation speeds of galaxies, right?
When those objects are becoming further separated, doesn't the attractive force of gravity have less of an influence, and therefore the objects don't get flung out? Its centrifugal force, not the expansion of space that would be causing the stars to get flung out.
Centrifugal force is most commonly introduced as an outward force apparent in a rotating frame of reference. It is apparent (fictitious) in the sense that it is not part of an interaction but is a result of rotation — with no reaction-force counterpart. This type of force is associated with describing motion in a non-inertial reference frame, and referred to as a fictitious or inertial force
reply to post by brazenalderpadrescorpio
I don't think I'm saying that. From what I understand dark matter is a form of matter that we have not been able to detect. Because it is matter, however, it does create gravitational attraction like visible matter. It's my understanding that dark matter is currently needed as a hypothesis precisely because the visible matter that we can see cannot account for various instances of gravitational attraction, especially at the large scale of galaxies attracting each other in galaxy clusters.
reply to post by ArtemisE
There is at least one physicist (you'd have to look it up; trust me, it's not something I've made up) that says that mathematics may be a mental construct (that is, exclusive to human thought). If that's true, then everything is subjective.
Edit: Here's just one link.
edit on bWed, 05 Mar 2014 01:50:02 -0600am63America/Chicago3amWednesday05America/Chicago by brazenalderpadrescorpio because: (no reason given)
This is true. Math is the secular mind's version of God. A perfect order that's uncontaminated by contextual "dirt". The problem is that Reality is comprised of contextual contamination and "dirt". Math is therefore useless as a theoretical tool. It's only useful if you know all the contextual sets involved and can accurately factor them into your equation.
Reality, as a whole, changes per Quantum instant of Now, which makes building the sets you'd need to factor into your equation impossible to determine.
I always get frustrated when math is used to try and "proof" a Reality theory