What is the Jimmy Kimmel / Matt Damon feud all about ?

The past few months I've been recording Jimmy Kimmel's late night show - I think he's the funniest of all the lame talk show hosts on at night. He keeps making digs at Matt Damon ; apparently they've had some kind of falling out from way back ? Does anyone know the details of what happened ?

It’s a running gag. They’re good friends.

It's because Matt has a huge head.

Thanks R1. I truly thought there was something going on between them. Kimmel is very convincing.