The atmosphere around our galaxy is nothing but vacuum. Light travels faster than anything but in space if there is no medium, then how come will light ever be able to travel. In case like this we are unable to determine what exactly happens but what we could do is just assume that this could happen.
So the conclusion is that if anything assume you can't be certain of, however,you give that certain percentage or that could happen. Well a common assumption is a more than 50% or less than 50% chance. Theoretically speaking more than 50% favorable chance of happening that thing and less than 50% of chance means there is a good chance of not happening that thing.
In reality that's not true if there is even .0001% chance of happening something it could happen.
