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There is an interesting article in the current S&T mag. regarding open clusters. The Pleiades is only about 125 million years old and there are few exoplanets found. The reason seems to be that planet migration is still in progress and may have a long ways to go. [Most exoplanets found are in much older open clusters where strong inward migrations likely took place.]It is particularly interesting that suggestions have been made that Oort Clouds may overlap between Solar Systems, even between the Sun and nearby Centauri systems at about 4 light years distant viz radius 2 light years. When stars are as close as 1 light year apart, considerable overlap might well be mandatory. Thus objects such as Oumuamua would be readily explicable. Such objects, as shared Oort Clouds, might form a continuum - thus 'clouds' might well be considered as continuously occupying large parts of galaxies.
If the estimate is correct for the Oort Cloud mass at ~ 0.01% of the Sun's mass, then it seems unlikely it will be any sizeable contributor to the DM estimate.If there are a few huge stars, more mid-size stars, and kazillions of small stars, it follows that there are kazillions squared planets, smaller planets, and even more smaller bodies, all out there somewhere. Could that be some of the dark matter, just stuff not lit up by stars?