Dear friend, Back is sometimes synonymous with ago, as in these sentences you question: We met a few minutes back/ago. ( Please note that meet requires an object or a plural subject, so I met few minutes back is nonstandard); It happened two days back/ago. Back, however, is impossible in the second sentence since it contains a lexical collocation - long ago.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.