A Matter of Fact:

Matter makes up everything around us, the illusion of how solid an object is or how transparent it is has to do with how the matter is arranged in the object.  Things with more distance between the matter that make up the object will seem to be less dense than those with the matter closer together.

 

So as space is created between the matter an object appears to become more transparent or even invisible to the eye as in the case of the air around us.

At some level in Taiji you stop thinking in terms of solid and transparent and move on to creating space, first with sinking and shifting then to the point of actually moving around some matter to create that mystical room or space that does not seem to be there for you to move in and out from but yet it appears.

Looked for can not be seen

Touched can not be felt

Listen for can not be heard