"It has stopped raining."
"yağmak" as a verb requires some sort of precipitation. If you want to use it, you must first make it into the gerund "yağma" and combine it with your form of precipitation (in this case, yağmur) "yağmur yağma." However, this is suddenly a noun compound and you need the 3rd person possessive ending for it to make sense "Yağmur yağması."
This literally would mean something like "The raining stopped" :)
Doesn't -miş just mean you didn't see it? Like if you looked out the window and it was raining, and looked again after 10 minutes and found it had stopped, you could say "Yağmur durmuş." The English present perfect ("has stopped") can play the same role because it means the event happened at an unspecified time.