What are the chances of Bernie winning the election by write-in votes?

I confess to being politically challenged, so please educate me on write-ins.

I did a tiny bit of research and found no presidents who won the actual election by write-in, but several who won the primaries by write in, including Eisenhower and JFK.

Would it be even remotely possible to win the actual presidency through a write-in, or would it default to a vote for Trump?