From what I understand the glass microspheres are made by using a blowing agent that decomposes into a gas at high temperatures expanding the glass into a hollow sphere-shell. As far as I am aware glass microspheres are used as a light weight filler that has a higher compression strength than alternative foams.

I expect that different manufacturers may use different blowing agents, meaning that some hollow microspheres may lack interior gasses due to the decomposition process being reversible at a lower temperature. However I could be wrong because glass microspheres can be manufactured with different blowing agents, and using different glass types.

Thank you very much on your reply. I supposed they are filled with air but your reply is interesting