We had an outer screen shatter one day. I was picking up a pen from underneath my seat at the time, and the impact of the object hitting our screen sounded like the PF had dropped a heavy manual onto the cockpit floor.
There were both straight and circular cracks, radiating out from a single point near the top of the screen. We were at FL350 with nothing above us - except possibly military aircraft, that we didn't know about.
I think it was something very small from outer space or possibly a paint fleck or tiny piece of debris from a satellite launch, which had accelerated to very high speed entering our atmosphere. We certainly didn't see what the object was.
I would post a photo of the screen but I cannot find it in my library.