but what is it exactly?
That MikeG is a very good question and thus far words like '
exotic matter get used quite a lot but it's really just a front for them not knowing.
I, in my humble, non expert opinion, hold that it may be a form of carbon left over from stellar formation (a sort of intergallactic
soot as it were) and is abundant throughout the universe, and in especially significant quantities between galaxies.
But then again I'm no experet.

However its effect on any spacecraft would, I'm pretty sure, be minimal as its distribution would be such that its density would be minimal and almost certainly energy nuetral, with a phyiscal temperature at the same level as the universal background.