What holds the galaxy together?

Main Question or Discussion Point

Just recently, I heard that at the speed the galaxy is currently spinning gravity alone is not a strong enough force to hold everything together. If that is true, what is the extra force that keeps the galaxy together?
 

Answers and Replies

mgb_phys
Science Advisor
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What you probably heard was that the mass of the galaxy estimated from counting the visible stars is not enough to keep it spinning at the measured speed.
The proposed solution is that there is extra mass in the galaxy in the form of dark matter which we can't see but has enough 'weight' to provide the extra gravity
 
thanks! even though i don't know much about dark matter, that really does help allot.
 
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i don't know much about dark matter,
It's okay, neither do the experts. ;)

Well, that's a lie, they know more-less where it should be, how much of it should be there, and what are some likely candidates for it, but what makes it up isn't sure yet. Lots of experiments being done to figure that out.
 
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Dark matter has never been detected in a laboratory. Names for particles which could make up dark matter go by exotic names, such as MACHOs (MAssive Compact Halo Objects), WIMPs (Weakly Interacting Massive Particles), and more exotic things like neutralinos etc.

Experimental high energy physicists are particularly keen to find new particles which could make up the dark matter content of the universe.

Alternate theories posit that dark matter does not exist, but that gravity is incorrect for very small accelerations (effectively, there is a minimum possible acceleration in the universe which is non-zero). These theories are collectively referred to as MOND (modified newtonian dynamics), although no generative theory exists yet, and experimental evidence is rather against it.
 

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