The more recent theories about the existence of dark matter are attempts to answer questions raised by findings that do not fit Newtonian or Einsteinian descriptions of gravity. A new paper proposes that gravity is variable. Stars that are moving too fast at the edge of a galaxy to be held to the galaxy by the current concepts of gravity are held by an increasing force of gravity. This suggests to me that the proposal a different 'unknown' force is making the universe expand at a faster and faster rate may be true.
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> http://www.newscientist.com/article/mg20327164.200-phantom-menace-to-dark-matter-theory... 
> Please excuse the hyperinflated editorializing in the headline. - This is great fuel for the debate on whether or not Dark Matter actually exists. - Good find!

I've come across this theory before, but it's nice to know there are now tests being worked out to decide the issue. That makes good science. However, I find the theory unsatisfactory, since the point at which the law of gravity changes seems quite arbitrary. There ought to be some reason for the change. But on the other hand the dark matter theory is also unsatisfactory, though for different reasons, namely that it requires undiscovered particles with odd properties. All we can say is: continue to "watch this space"!

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GPJ