My understanding is that strange matter is stable in low pressures or vacuum, but correct me if this is wrong
I'm wondering how likely is to find strange matter in empty space. What physical processes in decaying stars (supernovae, magnetars, etc.) can throw strange matter in outer space?
Finally, what are the most likely places to find strange matter in the universe? will most of it sink into the cores of stars and planets?
You should start by reading the Wikipedia entries on strange matter and strangelets. Witten's original paper is at http://prd.aps.org/abstract/PRD/v30/i2/p272_1 - annoyingly it doesn't seem to be on the arxiv.
Assuming strange matter is more stable than regular matter you wouldn't find it in normal stars because it would convert the entire star to strange matter and you end up with a strange star/quark star. You'd only find macroscopic quantities of it in strange stars. In that case you'd only find lumps of it floating around if they had been ejected by strange star collisions. Whether strange stars can form supernovae, and eject strange matter that way, I don't know and I'm not sure that anyone knows.
Strangelets could be formed in high energy collisons, so in principle they could be wandering around.
As I understand it, a chunk of strange matter colliding with a planet would pass straight through it and emerge slightly heavier having converted some of the matter in the planet on the way through. It would be hard for a chunk of strange matter to shed enough energy to settle at the core of a planet or star. In this respect it behaves a bit like a small black hole. If it did settle at the centre of a planet it would eventually convert the whole planet to strange matter.
But, but, but, but, all this is fun but extremely speculative. There is no experimental evidence for strangelets or strange stars, which is probably just as well.
Strange matter could also be created by neutron star collisions because of the extremely high pressure inside after which it would get thrown into space.
