DarkSky seemed to have their own weather service (not sure the source data), but they had their own API that other apps used. Apple shut the API down after a couple years. This meant the death of the DarkSky website and the old apps.
Apple says they use data from the weather channel, but this varies based on country. It used to say right in the app, but it seems like they removed that in favor of this link:
Apple says they use data from the weather channel, but this varies based on country. It used to say right in the app, but it seems like they removed that in favor of this link:
https://support.apple.com/en-us/105038