Who are the 144,000 in Revelation? Why is the Tribe of Dan Missing in the Book of Revelation? Reading Revelation 7 #341

Two end-times mysteries today. #1 What happened to the tribe of Dan - who is not listed in the list of the tribes of Israel sealed in Revelation 7. More controversially - who are the 144,000 mentioned in the book of Revelation? Are they faithful and elite Jehovah's Witnesses?? No!