Stolidog

It seems nonsensical that the earths atmosphere is held to it by gravity which is still an unproven theory when there is no apparent barrier keeping it from being sucked out to space which is a vacuum! Can somebody explain to me how the atmosphere stays around the earth. Also I'd like the answer to somehow include the laws of density and buoyancy as this obviously would be the alternative explaination to gravity should it be proven to not exist after all and that density and bouncy explains all things up and down.