Ask HN: N deG Tyson's “Origins”: Universe cannot be stable?

I've been reading Origins, by Neil deGrasse Tyson, and I have questions. He says that general relativity says that the universe as a whole cannot be stable - it has to be either expanding or contracting. (Note that we're not talking about the matter in the universe - the universe itself has to be expanding or contracting.

Can someone ELI20 why this is so?