"Gravity" is that magical word that they use to describe something when they don't have and answer for or can't create a sufficient lie that even the most indoctrinated would call bullshit. If they throw "gravity" out there the indoctrinated eat that shit up. You can use that word to describe anything they want even though they can't quite prove exactly what it is. People will even go so far as to believe without question that gravity is an excellent container.