I have no breaker space in the main 200amp panel. I'd like to set a 100 amp sub panel with its own breaker. Can I connect the sub panel to the main entry lugs on the 200 amp panel?
I'll tell you with 100% certainty that you CANNOT do this. Period.
It is a serious code violation and safety issue to have the new 100A feeder be unfused.
You have 2 easy solutions. You can pull two breakers from the main panel, and relocate them to the sub. Or you can replace 4 breakers with half height breakers to make room and free up the connection to the new panel.