Recommend an element use to fill bottles that contain ancient paper. the element should be a gas at room temperature, should be denser than helium, and should not easily react with other elements.

Okay so we are given these requirements:

element which can be used to stuff bottles that enclose ancient paper
must be a gas at room temperature
must be denser than helium
must not react with other elements

The only element that comes into my mind is:

Argon